Added context support

[lelandrichardson]

to: @ljharb @goatslacker

Adds support for context with mount and shallow.

The API looks like:

const context = { name: 'foo' };
const wrapper = mount(<Component />, { context });

const context = { name: 'foo' };
const wrapper = shallow(<Component />, { context });

Additionally, there is a .setContext(newContext) prototype method on each wrapper class.

Fixes #49

Note: this will constitute a minor change (new feature, backwards compatible)

[ljharb]

Is this guaranteed, ie, will it throw an exception? I don't see any tests that cover this case.

[lelandrichardson]

good call. will add some tests.

[lelandrichardson]

@ljharb tests added. comments addressed.

[ljharb]

Changes LGTM - @goatslacker should weigh in on the context stuff tho since he's got more experience with it than I

[lelandrichardson]

Looking at the coverage info with the line numbers mismatched is a PITA. I'm going to submit a followup PR that fixes istanbul line numbers, as well as improve coverage where needed

[goatslacker]

You're mutating spec here. Is that ok? Seems like a bad idea

[goatslacker]

Yeah it's ok.

[goatslacker]

LGTM

[blainekasten]

not due to this diff, but the airbnb styleguide shows this closing wrapper to be back 2 spaces. https://github.com/airbnb/javascript/tree/master/react#alignment

[goatslacker]

tru dat

[kujon]

Super useful PR. May I ask what is the reason for render not having this functionality?