Add option to specify SSH command (a-la rsync)

This is a continuation of #29.

The good news: I was able to work on this earlier than expected due to a personal project becoming a work project.

The bad news: I had to do this on a completely new workstation, and there were "difficulties" that required me to close the old pull request.

What I basically did was to implement the same option syntax as with rsync. Please do let me know if there are any problems or suggested changes.