Make warning (e.g., propTypes) behavior configurable

[sophiebits]

The implementation of the warning module in this repo is to do a console.warn. In most cases, you want this to be louder in development so that the warnings (really errors) are impossible to ignore. There should be a way to configure the warnings to produce an alert() or similar.

Relatedly, people should be able to easily set up their unit tests to fail if any propTypes checks fail.

[sophiebits]

This is close enough to #1753 that I'll close this one.