Persist map extent between sessions (via local storage)

[stdavis]

This is leftover from #212. There is also the idea of persisting map state via the URL which makes them shareable. Should we implement both?

[steveoh]

I could get behind a share button that serializes the app state for sharing purposes. I am not a huge fan of persisting that state between sessions though.

Maybe that could be configurable or I just deal with it.

[steveoh]

#62