Add option "--multi-fs"

[clvz]

This allows rsync to cross filesystem boundaries, which is useful
for my fancy partitioning scheme :)

[coveralls]

Coverage decreased (-2.3%) to 90.819% when pulling d914fff on clvz:master into 3f16a50 on xolox:master.

[xolox]

Hi Frédéric, thanks for the suggestion and sorry it took so long to reply:

• I've merged 518df84 and released rsync-system-backup 1.1 with the new option.
• I'm not going to merge the other commits in your branch, because I don't like the implication of eventually mirroring half of the rsync command line interface in the rsync-system-backup command line interface. I think that instead I will add something like --rsync-option to allow low level customization (see also More Flexibility (and Docs) #5).

I'll keep this pull request open until --rsync-option is available.