Pass the --use_strict flag through to mocha

Passes the --use_strict flag through to support harmony/ES6.

Adds a test to ensure that it works (only verifies a usage of 'let' but that should be enough).
The test is goofy because should.js isn't --use_strict compatible so the rule needs to remove test/mocha.opts for its run and return it after completion.

Requires mocha be clean with respect to --use_strict; added two vars to do this. The test will ensure it stays this way.