Add single file-type filter

[i3zhe]

Hi protocool,

I often want such a feature, that only search from some single file types, i.e. just from .rb .js or .css files. here rb doesn't include .rhtml .rjs .rxml .erb .rake .spec . I know there's --ruby liked options, but sometimes people just need a simple filter.

So I add these commits, please check to see if it's good to use. The wiki page is https://github.com/i3zhe/ack-tmbundle/wiki/File-Type-Filter