Add --all-partitions

[avh4]

This implements #53, by adding a --all-partitions option that when used will backup all found partitions as gzipped raw images.

However, I'm not sure if this should be merged as-is because there are several problems with this currently:

• --all-partitions says in the help that it implies --nandroid which is not actually correct; it simply causes all partitions to be backed up as gzipped raw images, and does not do other things that --nandroid does (like change the name of the backup folder that's created)
• it doesn't work with the --twrp mode where it will backup mountable partitions as tar files
• there's no way to exclude specific partitions (and it does not honor -U -E -S -B)