Clearify how to remove backups after a while

[roelvanduijnhoven]

This solves #63.

While attempting to build this I found out that this is actually already supported! If you supply 0 that will just work :).

What I did nevertheless:

• Included a test script that asserts it actually works as expected.
• Also introduced x and X as valid characters alongside 0.
• Updated the documentation to reflect how to use this.

Would love some feedback.

[roelvanduijnhoven]

@laurent22 Got any time to look at this?