Added visual feedback when the player selects a color during his/her turn.

- Changed drawColors(off) to drawColors(color) where color is an optional argument that can be either 'none' (for no colors highlighted), 'red', 'yellow', 'green', or 'blue' (to display any of these colors). If called with no arguments, the color at simonColors[colorIndex] will be highlighted.
- Added colorHighlighted variable to determine if a color is highlighted already to take appropriate action on mouseup and mousedown events as far as giving visual feedback goes.