Implement handling for provide/inject functionality

[tom-kitchin]

Not sure if you want me to write an issue when I've submitted a PR already, but just in case:

Vue 2.2+ includes the provide/inject functionality for a sort of component-level dependency injection. Could be faked in the existing vue-unit build by mounting the component to be tested within another component using build() to set up the provider, but it'd be nice to be able to stub out provides as part of mount().

So, PR with that functionality and tests at #5

[wrseward]

Thanks so much!