Move autocommands to plugin/go.vim. Fixes #238

Beside having fixed some issues, #232 introduced a new one (my bad...). Every time a go file gets edited, `ftplugin/go.vim` is sourced. The autocommands are cleared and defined again. The problem is that now the autocommand that execute `go#fmt#Format` works only for the last go file edited. To fix that, this patch moves autocommands to `plugin/go.vim`.