Added support for include and exclude of repos from command line

Hi Walter.
I got around to implementing the previously suggested --include and --exclude arguments as well.
The README is updated and should hopefully explain the usage, but in short:
If --include and/or --include-file are specified, the list of repos is taken from these, otherwise we do the default and look for any git-repo subfolders from current.
Either way, we then remove any repos specified by --exclude.
Hope it makes sense.
/J