Support 3 digit hex values

[LukasWieditz]

While usually a minor inconvenience to convert, generally hex values can be written as 3 digit codes when all three values (red, green and blue) have the same double digits, e.g. #ff00aa can be rewritten as shorthand #f0a. While not commonly used, it would be nice to have support for this to avoid having to handle this edge case manually.

[LukasWieditz]

This is implemented with #54