Allow '.' as Separator in Options and Arguments

[tgockel]

The paradigm used for "advanced" options at my company uses namespace-qualified option names on the command line. A la:

java -jar my-program.jar --com.foo.bar=100 --com.foo.baz=taco

Specifying these as options will always throw IllegalArgumentException, as this does not pass the OPTION_NAME_RE pattern in ArgParser.kt. Is there any reason to prevent using '.' as part of an option or argument (sibling to the '_' and '-' characters)?

[tgockel]

Also made a PR because the fix seems super easy. Not sure if disallowing '.' characters was intentional or just not done.

[xenomachina]

I was reserving the dot for a future feature I've been planning, but after thinking on it, I believe it should be fine to allow it in the way your PR enables.

Thanks for the suggestion, and the PR!