Enhancement: Add `--exclude-caches` and `--exclude-if-present`

A new option `--exclude-caches` was added that allows excluding cache
directories (that are tagged as such). This is a special case of a more generic
option `--exclude-if-present` which excludes a directory if a file with a
specific name (and contents) is present.

https://github.com/restic/restic/issues/317
https://github.com/restic/restic/pull/1170
https://github.com/restic/restic/pull/1224
