Cannot load text correctly by CsvParserPlugin if it includes `\0`.

Hi maintainers,

We sometimes encounter a problem CSV Parser cannot parse string containing `\0` .
This seems to be caused by CSV Parser judging that `\0` is the end of a sentence.

https://github.com/embulk/embulk/blob/45aa8fc1520cb43f9285b161cb1d033c90cc33fd/embulk-standards/src/main/java/org/embulk/standards/CsvTokenizer.java#L24

How can we avoid this problem? or Could you solve the problem?

See the following code to reproduce the problem.

```
$ ruby -e '5.times {|i| puts "#{i}\taaaaa\0aaa#{i}a\tname#{i}"}' > data.tsv
$ cat data.tsv # the below text includes `\0`
0 aaaaaaaa0a name0
1 aaaaaaaa1a name1
2 aaaaaaaa2a name2
3 aaaaaaaa3a name3
4 aaaaaaaa4a name4

$ cat config.yml
in:
type: file
path_prefix: ./data.tsv
parser:
type: csv
delimiter: "\t"
skip_header_lines: 0
columns:
- { name: id, type: long }
- { name: description, type: string }
- { name: name, type: string }
stop_on_invalid_record: true
out:
type: stdout

$ embulk run config.yml
2017-05-16 13:24:04.405 +0900: Embulk v0.8.21
2017-05-16 13:24:06.004 +0900 [INFO] (0001:transaction): Listing local files at directory '.' filtering filename by prefix 'data.tsv'
2017-05-16 13:24:06.006 +0900 [INFO] (0001:transaction): "follow_symlinks" is set false. Note that symbolic links to directories are skipped.
2017-05-16 13:24:06.010 +0900 [INFO] (0001:transaction): Loading files [data.tsv]
2017-05-16 13:24:06.055 +0900 [INFO] (0001:transaction): Using local thread executor with max_threads=16 / output tasks 8 = input tasks 1 * 8
2017-05-16 13:24:06.062 +0900 [INFO] (0001:transaction): {done: 0 / 1, running: 0}
2017-05-16 13:24:06.121 +0900 [INFO] (0001:transaction): {done: 1 / 1, running: 0}
org.embulk.exec.PartialExecutionException: org.embulk.spi.DataException: Invalid record at line 1: 0 aaaaaaaa0a name0
at org.embulk.exec.BulkLoader$LoaderState.buildPartialExecuteException(org/embulk/exec/BulkLoader.java:373)
at org.embulk.exec.BulkLoader.doRun(org/embulk/exec/BulkLoader.java:591)
at org.embulk.exec.BulkLoader.access$000(org/embulk/exec/BulkLoader.java:33)
at org.embulk.exec.BulkLoader$1.run(org/embulk/exec/BulkLoader.java:389)
at org.embulk.exec.BulkLoader$1.run(org/embulk/exec/BulkLoader.java:385)
at org.embulk.spi.Exec.doWith(org/embulk/spi/Exec.java:25)
at org.embulk.exec.BulkLoader.run(org/embulk/exec/BulkLoader.java:385)
at org.embulk.EmbulkEmbed.run(org/embulk/EmbulkEmbed.java:180)
at java.lang.reflect.Method.invoke(java/lang/reflect/Method.java:498)
at org.jruby.javasupport.JavaMethod.invokeDirectWithExceptionHandling(org/jruby/javasupport/JavaMethod.java:453)
at org.jruby.javasupport.JavaMethod.invokeDirect(org/jruby/javasupport/JavaMethod.java:314)
at RUBY.run(/Users/takahiro.nakayama/bin/embulk!/embulk/runner.rb:84)
at RUBY.run(/Users/takahiro.nakayama/bin/embulk!/embulk/command/embulk_run.rb:269)
at RUBY.(/Users/takahiro.nakayama/bin/embulk!/embulk/command/embulk_main.rb:2)
at org.jruby.Ruby.runInterpreter(org/jruby/Ruby.java:850)
at org.jruby.Ruby.loadFile(org/jruby/Ruby.java:2976)
at org.jruby.RubyKernel.requireCommon(org/jruby/RubyKernel.java:963)
at org.jruby.RubyKernel.require(org/jruby/RubyKernel.java:956)
at org.jruby.RubyKernel$INVOKER$s$1$0$require19.call(org/jruby/RubyKernel$INVOKER$s$1$0$require19.gen)
at RUBY.(root)(uri:classloader:/META-INF/jruby.home/lib/ruby/stdlib/rubygems/core_ext/kernel_require.rb:1)
at Users.takahiro_dot_nakayama.bin.embulk.embulk.command.embulk_bundle.invokeOther66:require(Users/takahiro_dot_nakayama/bin/embulk/embulk/command/file:/Users/takahiro.nakayama/bin/embulk!/embulk/command/embulk_bundle.rb:51)
at Users.takahiro_dot_nakayama.bin.embulk.embulk.command.embulk_bundle.(file:/Users/takahiro.nakayama/bin/embulk!/embulk/command/embulk_bundle.rb:51)
at java.lang.invoke.MethodHandle.invokeWithArguments(java/lang/invoke/MethodHandle.java:627)
at org.jruby.Ruby.runScript(org/jruby/Ruby.java:834)
at org.jruby.Ruby.runNormally(org/jruby/Ruby.java:749)
at org.jruby.Ruby.runNormally(org/jruby/Ruby.java:767)
at org.jruby.Ruby.runFromMain(org/jruby/Ruby.java:580)
at org.jruby.Main.doRunFromMain(org/jruby/Main.java:425)
at org.jruby.Main.internalRun(org/jruby/Main.java:313)
at org.jruby.Main.run(org/jruby/Main.java:242)
at org.jruby.Main.main(org/jruby/Main.java:204)
at org.embulk.cli.Main.main(org/embulk/cli/Main.java:23)
Caused by: org.embulk.spi.DataException: Invalid record at line 1: 0 aaaaaaaa0a name0
at org.embulk.standards.CsvParserPlugin.run(CsvParserPlugin.java:368)
at org.embulk.spi.FileInputRunner.run(FileInputRunner.java:156)
at org.embulk.exec.LocalExecutorPlugin$ScatterExecutor.runInputTask(LocalExecutorPlugin.java:294)
at org.embulk.exec.LocalExecutorPlugin$ScatterExecutor.access$000(LocalExecutorPlugin.java:212)
at org.embulk.exec.LocalExecutorPlugin$ScatterExecutor$1.call(LocalExecutorPlugin.java:257)
at org.embulk.exec.LocalExecutorPlugin$ScatterExecutor$1.call(LocalExecutorPlugin.java:253)
at java.util.concurrent.FutureTask.run(FutureTask.java:266)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
at java.lang.Thread.run(Thread.java:748)
Caused by: org.embulk.standards.CsvTokenizer$TooFewColumnsException: Too few columns
at org.embulk.standards.CsvTokenizer.nextColumn(CsvTokenizer.java:168)
at org.embulk.standards.CsvTokenizer.nextColumnOrNull(CsvTokenizer.java:379)
at org.embulk.standards.CsvParserPlugin$1.nextColumn(CsvParserPlugin.java:346)
at org.embulk.standards.CsvParserPlugin$1.stringColumn(CsvParserPlugin.java:302)
at org.embulk.spi.Column.visit(Column.java:58)
at org.embulk.spi.Schema.visitColumns(Schema.java:81)
at org.embulk.standards.CsvParserPlugin.run(CsvParserPlugin.java:259)
... 9 more

Error: org.embulk.spi.DataException: Invalid record at line 1: 0 aaaaaaaa0a name0
```