Add a custom submit script.

This should set the custom submit command to allow the pre-submisison magic to occur (similar to how it does for cyclus).

@scopatz - how does this get to the polyphemus server?  Is it pulled every time a PR causes a ping, or is this a manual update?  (Also, does it need it's own repo, or could it be squirreled away in the dagmc-ci repo?)