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

Wouldn't `del args[index]` be more efficient? `args.remove(arg)` first has to determine the index again.