Create 'exec' filter

Migrated from https://logstash.jira.com/browse/LOGSTASH-119:

> Would be useful to pipe arbitrary fields through a command to modify them.
>
> Here's an example that would anonymize hostnames or something.
>
> ```
> filter {
> exec {
> command => "sed -re 's/\S+\.loggly\.com/anonymizedhost.example.com/'"
> fields => [ "@message", "hostname", "@source_host" ]
> }
> }
> ```
>
> The default would use only the `message` to parse
>
> The protocol between logstash and the exec filter must be strict. Something like:
> for every line emitted, one line must be emitted as the 'new' line. If no changes are made, simply print it unmodified.
>
> deleting the field can be done by printing a blank line
>
> we exec the process once and use stdin for sending data, stdout for reading responses; if it dies, some retries should occur