dillon1000/react

Add custom eslint rule for warning and invariant

See #2869

Checks that the *second* argument of warning and invariant are a literal
string, or a concatination of literal strings, and that the number of
arguments is correct based on the number of %s substrings.

This commit also fixes a few places where the existing code had broken
error messages!

The rule itself is pretty straightforward, although adding the tests
ended up being a bit painful, as eslint-tester depends on mocha, and
therefore needs to be run in a separate grunt task.