Add 'id' attribute to require.context (so HMR can accept a context)

### Use case: I want to hot-module-reload templates.

In have created a `require.context` in my `Router.js` for all `jade` templates.

#### Example HMR update

1. I save a template. 
2. Template module is invalid. Propagates to parent.
3. Require Context module is invalid. Propagetes to parent.
4. Router accepts and re-renders template.

So, the `Router.js` has a `module.accept` call which needs the invalidated Require Context `moduleId`. I've added this as an attribute to the Require Context.

(PS. I first thought HMR should work if I accept all grandchildren. I thought: I just have to "catch" the invalidation when it bubbles up, and then HMR will work. However, the child could have side effects that make a HMR impossible, that is why **HMR can only accept direct children!!!!**. This is a huge insight and should be documented somewhere!!!!)