Serialize and cache state

[alukach]

It would be great if DropChop cached its state to the browser to be loaded between usages. This would minimize the pain of accidental reloads and allow users to return to their work if they need to close the app during usage.

To cache state, we will first need to be able to serialize DropChop's state (layers loaded, operations ran). This would be extremely useful if/when we move towards making DropChop states shareable (as outlined in #130, where we would just write the state to a gist or other third-party storage). This may be easier following the completion of #140.

An example of caching state to browser can be seen when using geojson.io, as it prompts a user to "recover your map from the last time you edited?" when you use the app for the not-first time.