Post & pre commands support

[hadret]

This PR fixes a lot of long standing bugs, but also provides the following features:

• pre_ and post_ commands can now be added to the jobs, like so:

 jobs:
   - command: 'restic-backblaze-example backup /var'
     at: '0 4  * * *'
     pre_command: 'mysqldump --all-databases > /srv/my.sql'
   - command: 'restic-backblaze-example backup /home'
     at: '0 3  * * *'
     user: 'restic'
     post_command: 'systemctl restart some-app'
   - command: 'restic-backblaze-example backup /var/or/sth'
     at: '0 5  * * *'
     pre_command: 'mysqldump --all-databases > /srv/my.sql'
     post_command: 'systemctl restart some-app'

Not specifying these jobs is fine and backwards compatible (closes #4).

• user has ben brought back and now can be specified in the job scope (otherwise it will fallback to the restic_user).
• quite some bug fixes in tests and test-related things; Travis CI should now also test against Rocky Linux 8.