Provide a way to serialize/deserialize Webamp

[captbaritone]

Some parts of Webamp will be hard to serialize (tracks, for example), but the rest of the app should be pretty possible to serialize, since we use Redux for pretty much everything.

Once we have a serializable state we could add a way to construct a new Webamp instance from that saved state. I'm not sure yet if this should be a static method: const webamp = Webamp.fromSavedState(savedState); or an additional property to the constructor options.

[captbaritone]

#658

[captbaritone]

#670

[captbaritone]

Fixed in #670