Mocha.ui() now tries to import ui module if name is not found in mocha.interfaces

Hi, `--require <ui-module> --ui <name of the ui exported by ui-module>` doesn't work cause requires processed after settings an UI. I also think that having a `--ui` option to require a module is also consistent with how `--reporter` works and more straightforward than using `--require mod --ui name` even if it would work.

I believe this is better a version of #851.