Add onChange callback to Playground

[janmyler]

Add onChange prop to <Playground /> to allow propagation of code changes outside of the component – this allows use cases such as storing the changes to localStorage as you type and restoring them in the future.

[kylecesmat]

@janmyler could you add documentation about this new prop? Seems like a good feature to me.

[nhunzaker]

@janmyler Thanks for implementing this! I'd like to use this feature to generate Github issues at the click of a button:

Any updates? Otherwise, I'm happy to take this over.