Allow passing of active breakpoints into the onBreakpointChange callback

Does pretty much what it says on the can...

The use-case for this is to allow for programatically taking action when a mediaquery changes, and having a reference for which breakpoints are actually active at the time of the event being triggered.

This is cool because it would allow you to have your client-side app code to consume and respond to the current breakpoint, for example by redrawing the UI using a system like React.

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### TODOs

- [x] Add example
- [x] Minify
- [x] Update README.

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![untitled mov](https://cloud.githubusercontent.com/assets/18076/3347688/8fd7362c-f8f3-11e3-926a-e1e24aa68585.gif)