Add Support for dependency injection

It would be really nice to support dependency injection.  On the command line, you could add a flag for a provider module path.  In a browser you could load a provider before your test scripts.  It would probably look like angular's factory pattern.

``` javascript
mocha.factory('someDep', function(){return a;});

//later in my test
describe(function(someDep){
//tests here
});
```
