HSL Support

[varlevi]

Is your feature request related to a problem? Please describe.
Not a problem, but an increasingly important feature as it seems lots of devs are using hsl color values.

Describe the solution you'd like
I'd like a way, much like with hex values, to input and output hsl values.

Describe alternatives you've considered
Link to an already-created site for converting color values (probably not a great idea!)

Explain any problems that could arise
See issue #53.

Additional context
The following stack overflow pages may be helpful:

HSL to RGB
HSL to Hex and RGB