Adds ability to load external interfaces

[itaylor]

This adds support for loading additional interfaces with the --ui option, similar to how the --reporter option works.

After this commit, you should be able use an alternate interface like this:
npm install someMochaUI
mocha --ui someMochaUI path/to/a/mochatest.js

[andreypopp]

Can we have it merged? I want to use a custom interface which wraps it blocks in fibers. mocha --require ./fiber-ui --ui fiber-ui doesn't work (I update mocha.interfaces inside ./fiber-ui.js but it seems to get the wrong instance of mocha module).

[andreypopp]

Not sure though if it should mutate exports.interfaces global, maybe just setting this._ui?

[itaylor]

I managed to work around the lack of this feature by making sure that I got the right instance of the Mocha object. I did this by marking it in the peerDependencies instead of dependencies in package.json then grabbing the Mocha instance from module.parent.require

That allowed me to add my interface in to the running instance of mocha.
Here's a commit that illustrates doing this. itaylor/qunit-mocha-ui@125201c

After that change I was able to add a new interface to the Mocha.interfaces object, and it was there in the testrunner's Mocha instance.

I hope that helps you out some, although it'd be a lot easier this feature would get merged, though.

[andreypopp]

@itaylor see #1022 — it appears requires processed after setting an UI so even if I get right mocha instance this won't work

[travisjeffery]

gonna go with #1022