add test: should underscore periods for numbers

Use-case: CSS ID can't have [unescaped period](http://jsbin.com/mimey/1/edit), and places where you want to revert `1_2_10` back to `1.2.10`.  I'd also prefer this for URLs.

I'm not sure if spaces surrounding numbers should be underscores also.

My RegEx skills are weak, but my copy-paste-tweak skills are strong :)