Ignore <session>x.vim files in session path

These files can be used to set up additional things for a session and have to
be placed in the same directory as the session file.

See point 10 under `:h :mksession` for more information.

Since they are no real session files, Startify shouldn't list them either.

Closes #179.

autoload/startify.vim

@@ -494,7 +494,7 @@ endfunction
 
 " Function: s:show_sessions {{{1
 function! s:show_sessions() abort
-  let sfiles = split(globpath(s:session_dir, '*'), '\n')
+  let sfiles = filter(split(globpath(s:session_dir, '*'), '\n'), 'v:val !~# "x\.vim$"')
   if empty(sfiles)
     if exists('s:last_message')
       unlet s:last_message