Allow importing/exporting progress

[sharmaeklavya2]

It will be useful to be able to import/export the progress of the game, i.e. which items are packed at what places in the bins. This can be used to save a game and resume later. We can do this by serializing to CSV or JSON.

~We should also allow exporting to SVG/TikZ/PNG, because this game is a good way to create packing diagrams. This would be especially helpful if we allow a user to easily choose the input instance.~ (See #15 for a related issue.)

[sharmaeklavya2]

• Run var pos = globalGame.getItemPositions(); in the developer console. This will store the positions of packed items into the variable pos. Later, you can use globalGame.putBack(pos) to restore progress.
• Run downloadProgress(); in the developer console. You will then be prompted to save your progress as a JSON file. Later, you can run loadGameFromUpload(); to upload a JSON file to resume your game from. Alternatively, you can drag-and-drop a JSON file onto the browser window.

~Pending: exporting to SVG, TikZ.~