Allowing to set configuration file's name on command line

Hi,
this is my implementation for allowing to set the configuration's file name (Issue #1501) with the command line argument --conf=xxx. The argument is removed from the command line to avoid problems later-on when commands or plugins try to evaluate the argument.
Best,
Felix

I wasn't able to get it to run with _reload, that's why I removed it. I'm also assuming that when loading the .py files explicitly via imp/importlib, this shouldn't be necessary anyway.