Support requiring assertion libs

[geddski]

Mocha's CLI allows you to pass a --require option for including your assertion library (should, chai, expect etc) so you don't have to require it in every single test.

Mocha's programmatic interface currently doesn't do anything with the "require" option. This pull request shims in support for loading your assertion lib by setting the require option. In the future if/when Mocha supports this option, we can drop these few lines and stay completely backwards compatible.

Closes #11

[yaymukund]

You're just calling addFile. How about something like:

simplemocha: {
  all: {
    src: [
      // require should...
      'node_modules/should/lib/should.js',

      // This test can use it!
      'tests/my_test.js'
    ]
  }
}

I'm hesitant to add special code for should, chai, and expect. I'm with visionmedia that there's usually no need to encourage using a lot of globals.

[geddski]

Good call, that's simpler. Thx.

[steida]

Correction: 'node_modules/should/lib/should' should be 'node_modules/should/lib/should.js'