Add fmt command

[jhoobergs]

• create a rumbas fmt command to format all exams / questions the same way. Order of fields equal to order in rust code.

[jhoobergs]

Should do following things as well:

• print strings by default without quotes
• never allow double backslashes: format as multi-line in that case.
• look for maps with a content and a placeholder field and parse and write them as translatable strings (so it is shown as a string when possible)
• Keep comments

[jhoobergs]

#117 solved this partly. Comments are now being kept. There isn't much formatting yet.