commit: Handle archiving exams

Provide or adjust options to archive exams. If selected, the folder
referenced by klausurarchiv.config.defaultArchiveId from last commit
(a233a8e8) will be used as the default
archive folder. The user may also (optionally) select different archive
folders. Exams selected for archiving will not be printed.

See #91
Apart from the documentation, that issue is fixed now.