Suggestion: multiple SOURCEs

I'm very glad I found your project! - I like it very much.

please, enable multiple sources like rsync does:
`rsync SRC... [DEST]`

reasons:
I have multiple SOURCE dirs which I want to backup at the same time into the same TARGET. Currently, I have to issue one commandline per SOURCE
- Multiple required commands glued together into one line are ugly, but this seems to be the only way to start a multi-source `timemachine` backup from cron or other system event.
- a multi-source multi-liner produces an ugly result, because only the latest SOURCE remains in target _Current_ directory, while all other SOURCE dirs of my backub job are cluttered each into another _timestamped/past_ directory in the target.

Start by locating the command-line argument handling and the rsync-based backup entry point. Compare the requested `rsync SRC... [DEST]` form with the current behavior, including the `Current` and timestamped directories. Done means multiple source directories can be backed up together without leaving the target layout inconsistent.

Written by the indexing model from the issue text.