Add the go ctags parser

Make go one of the builtin filetypes, add the parser and update the related
source and config files. While there, remove Rust from [Groups] in
filetype_extensions.conf because it's already a builtin filetype as well.

The parser itself is stolen from the fishman/ctags repo.
Merge pull request #373 from techee/go_ctags

Add a Go ctags parser.