Only allow `[a-zA-Z0-9$_]` in JSON-P callback names

Ref. #31 and https://github.com/mezzoblue/csszengarden.com/pull/24/files#r4150859.
Merge pull request #31 from mathiasbynens/safer-jsonp

Only allow `[a-zA-Z0-9$_]` in JSON-P callback names