Add knapsack support

[arkaray]

The game can be easily extended to 2D knapsack.

[sharmaeklavya2]

Depends on #5.

[sharmaeklavya2]

How will we show the profit of items? We can use the bottom status bar to show the exact value when the item is clicked (like we currently show the dimensions), but given the limited memory of users, users need a way of quickly glancing the profit of all the items. Maybe we can color-code the items by profit-density, so small items with large profit will be darker/redder.