Added option to pass --authors-prog flag to "git svn fetch" command.

Thanks for the great utility. I needed the ability to pass the --authors-prog flag to handle SVN usernames that were not found in the authors file. Sadly, this flag doesn't have a git config key like svn.authorsfile, so you have to pass the flag again if you want to use it with the --rebase. Otherwise, it works great for me (tm). Thought I would contribute my changes back. Hope this helps.