Import / Export game data

[MikhaD]

Description
An option in the settings allowing the user to import or export their game data.

Reasoning
Because the game uses localStorage for data persistence it is local to individual browsers on individual devices. This would allow users to transfer their data should they wish to change device or browser and not have to start again.

Implementation (optional)
Serialize the game data into some form of lightly obfuscated base 64 string to make it a little harder to modify during transport. If a user really wants to cheat they can just change their data in localStorage.