Set the default completion type of proxy command to "file"

Though we do not know the completion type of a command until the command
is loaded, setting it to file seems like the best thing we can do.

plug.vim

@@ -251,7 +251,7 @@ function! plug#end()
 
   for [cmd, names] in items(lod.cmd)
     execute printf(
-    \ 'command! -nargs=* -range -bang %s call s:lod_cmd(%s, "<bang>", <line1>, <line2>, <q-args>, %s)',
+    \ 'command! -nargs=* -range -bang -complete=file %s call s:lod_cmd(%s, "<bang>", <line1>, <line2>, <q-args>, %s)',
     \ cmd, string(cmd), string(names))
   endfor