Append Go file type when creating temporary file

A lot of scripts filter files based on their name to make sure they
operate only on the appropriate content. By appending a Go file type to
a temporary file when running a formatter, we add support for a broader
range of Go formating tools.

Fixes #208

autoload/go/fmt.vim

@@ -58,7 +58,7 @@ function! go#fmt#Format(withGoimport) abort
   endif
 
   " Write current unsaved buffer to a temp file
-  let l:tmpname = tempname()
+  let l:tmpname = tempname() . '.go'
   call writefile(go#util#GetLines(), l:tmpname)
   if go#util#IsWin()
     let l:tmpname = tr(l:tmpname, '\', '/')