Add minimal go syntax, ftplugin and set filetype to go when open file *.go

From pull request [#1041](https://github.com/neovim/neovim/pull/1041). I send to review again.
Can you move this block to in between [Gkrellmrc (line 793)](https://github.com/wingyplus/neovim/blob/go_syntax/runtime/filetype.vim#L793) and [GP scripts (line 795)](https://github.com/wingyplus/neovim/blob/go_syntax/runtime/filetype.vim#L795)? That way it'd be in alphabetical order.