Custom attributes checker

[topisani]

Id like to write a clang-based custom static analysis tool, to add our own [[otto::attributes]]. These should be to enforce things like.

• [ ] Checking action handlers
• [ ] Make sure functions on the audio thread only do things functions on the audio thread are allowed to do.

And more...

This checker should then run on travis to be part of our regular CI

It might make sense to wait for c++20 to do this...