Make io.votable.exceptions.MAX_WARNINGS configurable

Currently, io.votable.exceptions.MAX_WARNINGS is hardcoded to 10. There might be cases where users wish to increase/decrease that value. Perhaps we should make it a configurable item in io/votable/__init__.py; but if that introduces ugly circular imports, it can instead be a configurable item within exceptions.py itself.

Whoever wants to tackle this also has to write tests for it (and a change log).