Add an environment variable for EXCLUDES_FILE

[Alb11747]

Sometimes when you need to exclude a lot of files having them all in one line can become unwieldy. I think an option should be added to support a excludes file. This is already implemented as option in rclone, rsync (--exclude-from), and restic (--exclude-file).

[itzg]

For the "tar" backup method right?

[Alb11747]

Ideally it should work with all backup methods. What I have basically just acts the exact same as EXCLUDES (#126). I haven't been able test it yet, though.