travis: check for doxygen warnings

This introduces a script that will check for doxygen warnings in newly committed or edited files.
I can't see something that makes `--diff-filter=MR` not fail. If that's the case: use one line without `--diff-filter=<foo>` because it will default to ACMR anyways.
On the other hand not hardfailing on `MR` is likely preferable, but in that case I guess the script needs adjustment.