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.

You can configure if data is compressed with the option `--compression`. It can
be set to `auto` (the default, which will compress very fast), `max` (which
will trade backup speed and CPU usage for better compression), or `off` (which
disables compression). Each setting is only applied for the single run of restic.
The option can also be set via the environment variable `RESTIC_COMPRESSION`.

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.

To upgrade in place run `migrate upgrade_repo_v2` followed by `prune`. See the
documentation for more details. The migration checks the repository integrity
and upgrades the repository format but will not change any data. Afterwards,
prune will rewrite the metadata to make use of compression.

As an alternative you can use the `copy` command to migrate snapshots: 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/issues/3779
https://github.com/restic/restic/pull/3666
https://github.com/restic/restic/pull/3704
https://github.com/restic/restic/pull/3733
