Support programmatic specification of requirements

Move support for "requirements" into the Mocha library itself (instead
of a CLI abstraction) so that requirements can be used programmatically
through the Mocha API.

I think it's generally a good idea to have this functionality available programmatically. My use case is with [the "Simple Mocha" plugin for Grunt](https://github.com/yaymukund/grunt-simple-mocha), where I'd like to specify Mocha test requirements from my Gruntfile.