API for warning messages

[yocontra]

Example: less emits a lot of errors as warnings, which is pretty annoying.

Proposed solution: Either convert warnings to errors, or add another API to listen for warnings.

[jescalan]

I think this is a good idea -- we can't just convert warnings to errors I don't think, but a warnings API seems appropriate. Unfortunately I don't have the time on my hands to implement this at the moment, but when I do have more I would love to return to this. In the meantime if someone else needs it I would love to see a PR!

[matthew-dean]

Less emits errors as warnings? Errors that cause it not to compile? If the compiler can complete, then it's a warning.