Add projectionlist for golang

 - In addition to having better support for :A, :AV, :AS, etc we now
 have :Etest, :Ego and :Esuite to quickly jump to golang files
 - This makes our golang support more exetensible as it uses tpope's
 projectionlist plugin (the same one used for ruby file jumping) and
 - Avoids accidental command collision with his bindings.
Background gotags generation

 - Uses vimproc#system_bg to background shell commands
 - This gives us a little more speed as gotags get generated, since you
 can still use vim while they're going
 - Will by default regenerate the tags whenever you save a file in
 golang, this is highly experimental and should be disabled if causes
 and problems: `let g:disable_gotags_on_save = 1` to disable it at any
 time on your `~/.vimrc.local.before` to disable this feature