Send the Coqtop stderr output to a pipe, as opposed to to Vim

This should fix issue the-lambda-church#61.

I tried doing what the code for Windows NT did and redirect the stderr
output to stdout, but that freezes my Vim process for some reason. So I
just send it to a pipe that is never read from.

autoload/coqtop.py

@@ -226,6 +226,8 @@ def restart_coq(*args):
                 options + list(args)
               , stdin = subprocess.PIPE
               , stdout = subprocess.PIPE
+              , stderr = subprocess.PIPE # Do this so that the stderr output doesn't randomly appear on the screen
+                                         # TODO: Maybe display the stderr output somewhere useful?
               , preexec_fn = ignore_sigint
             )