support for more detailed requires

Example usage:
```javascript
function requireChai() {
	expect = require("chai").expect;
}

function requireMain() {
	main = require("./lib/index.js");	// relative to Gruntfile
}

grunt.initConfig({
	mochaTest: {
		"single": {
			options: {
				require: requireMain,
				ui: "bdd"
			},
			src: ["test/*.js"]
		},
		"multiple": {
			options: {
				require: [requireMain, requireChai, "something-else"],
				ui: "bdd"
			},
			src: ["test/*.js"]
		}
	}
});
```