Improving the colour picker

[gitchly]

It might be better to implement https://jscolor.com/ to ensure more consistent cross-platform behaviour and compatibility, rather than relying on the current input type="color" HTML spec.

[gitchly]

After playing a few rounds and being frustrated at not being able to pick the same colours again, I'm considering making the game keep track of colour history so it's easy to switch to other colours you've used.