Canvas sizes

[Pamblam]

Someone added a drop down that allows the user to change the canvas size. It's a great feature but doesn't work as expected on all screen sizes.

Please:

• Check user screen size and only allow options smaller than the screen.
• Make sure canvas is always centered no matter what size is chosen.
• I think scrolling was disabled. Please re-enable it so scores can be seen.

[Pamblam]

Thank you @hjdesigner. Would you mind adding your name to the readme as well since you contributed.