Fixes issue #1.

Restic 0.9.3 fails when --stdin-filename contains slashes:

repository 954a4e61 opened successfully, password is correct
error: file does not exist
Fatal: unable to save snapshot: snapshot is empty

Covers the whole SourceSpec -> Dataset generation pipeline.

Prevents a panic when nesting WithStatus calls (for whatever reason).

Get rid of the Check(), do it all in Parse().

Handlers can now modify Datasets on-the-fly and influence the
structure of the generated backup jobs. Backup job creation is also
delegated to Handlers.