Enhancement: Add comppression support

We have added compression support to the restic repository format. To create a
repository using the new format run `init --repository-version 2`. Please note
that the repository cannot be read by restic versions prior to 0.14.0.

The new format version has not received much testing yet. Do not rely on it as
your only backup copy! Please run `check` in regular intervals to detect any
problems.

Upgrading in place is not yet supported. As a workaround, first create a new
repository using `init --repository-version 2 --copy-chunker-params --repo2 path/to/old/repo`.
Then use the `copy` command to copy all snapshots to the new repository.

https://github.com/restic/restic/issues/21
https://github.com/restic/restic/pull/3666