Implement `Show` for `Color`

This PR adds `Color::to_32_bit()` and `Color::to_hex()`. 

It also implements `Show` for `Color`, which outputs the result of `Color::to_hex()`.

Hexadecimal is a familiar representation for colors, but it does require clamping to `[0..255]` and rounding.

Closes #57. 

Open to suggestions. If a simple printing of the array is preferred, I will withdraw this PR, though the hexadecimal conversion might be useful.