Add context helpers

[suchipi]

describeComponent should expose setContext and context helpers that work the same as setProps and props, but for context.

[suchipi]

As of 1.1.0, you can pass options for enzyme's mount, shallow, or render functions into mountWrapper, shallowWrapper, or renderWrapper. This lets you set context like this:

describeComponent(MyComponent, ({ mountWrapper }) => {
  mountWrapper({ context: { some: "context", goes: "here" } });
});

However, the usage can be a bit awkward, because the context set in the first invocation of mountWrapper or shallowWrapper carries over to the next time mountWrapper or shallowWrapper is called (since the result is memoized). For this reason, I'd still like to eventually add setContext and context- but using enzyme's options is an easy workaround in the meantime.