Force wiki files to have filetype=vimwiki

Fix #122

autoload/vimwiki/base.vim

@@ -965,6 +965,9 @@ function! vimwiki#base#edit_file(command, filename, anchor, ...) "{{{
   " getpos() directly after this command. Strange.
   if !(a:command ==# ':e ' && vimwiki#path#is_equal(a:filename, expand('%:p')))
     execute a:command.' '.fname
+    if &filetype != 'vimwiki'
+      set filetype=vimwiki
+    endif
   endif
   if a:anchor != ''
     call s:jump_to_anchor(a:anchor)