Better error handling

Some people have been using this script for a while now -- without major hiccups. So I guess it's time for v1.0.0.

Improvements

• If you're using Pandoc v2.10 or newer, errors no longer abort
  document compilation and error messages are more to the point.
• Error messages are formatted more nicely.

Bugfixes

• The test suite can now test under 'real world' conditions again.
• warn no longers throws an error if it's called with arguments to msg.
• add_references creates a empty array instead of an empty object for references.

Varia

• Minor improvements to documentation.
• Data files of the test suite no longer get updated by testing.