Fix inability to handle a $VERSION, and insert comment on package line!

as discussed ages ago in https://github.com/rjbs/Dist-Zilla-Plugin-Git-Describe/issues/2.

I started thinking about doing this when I was buried in the [PkgVersion] changes and realized that I understood PPI enough to tackle this, then I made a few discoveries:

- this plugin had no tests!
- if a $VERSION line existed, we croaked! (looks like a bad copy-paste from PkgVersion, as it's in the very first commit)

I made the add-to-package-statement-line an option, but it's easy enough to yank the config and make it the only way it works.