Adding better support for .class and #id allowed chars.

I noticed that #_ does not highlight as an identifier. According to the specs underscore is allowed directly after # and the same rules apply to .class so I changed the regex for that as well to be the same as for #id. 

http://www.w3.org/TR/CSS2/syndata.html#value-def-identifier

Cases I did not cover in this change are
- allow single hyphen 
- allow single hyphen followed by a digit