Add file types searching like ack and ag

[rkulla]

Please add flags for searching by different types of files. For example, this works in ack and ag but not pt: Search for 'foo' in just ruby and go files pt foo --ruby --go.

To do this properly, also add a --list-file-types command like ack and ag have, so users can see at a glance that --ruby will search for .rb .rhtml .rjs .rxml .erb .rake .spec file extensions, etc.

The -G, --file-search-regexp achieves a different goal, and is fine to keep (even ag has that) but it alone is too painful for doing quick operations like this. I have to do these types of operations constantly and would really appreciate it.