Base output directory should strip non-word characters

For example:

```
$ sbt new file:///Users/tmoore/Projects/lagom/lagom-java.g8
[info] Loading global plugins from /Users/tmoore/.sbt/0.13/plugins
[info] Set current project to java (in build file:/Users/tmoore/Projects/tmp/java/)
name [Hello]: Hello, world!
organization [com.example]:
version [1.0-SNAPSHOT]:
lagom_version [1.3.1]:

Template applied in ./hello,-world!
```

I would have expected it to name the directory "hello-world" without the additional punctuation.

This could be fixed one of two ways:

1. Apply the "word-only" format to the "name" parameter [when constructing the base directory](https://github.com/foundweekends/giter8/blob/0.7.x/library/src/main/scala/g8.scala#L257).
2. Apply the "word-only" format to [the "normalize" format itself](https://github.com/foundweekends/giter8/blob/0.7.x/library/src/main/scala/g8.scala#L193) before hyphenating.

I think that the second option makes sense for an intuitive understanding of what "normalize" ought to do, but it might be an unexpected change for some people.

I'm interested in people's thoughts, and I can make a pull request with whatever the consensus is.