Add darken, lighten, blend, spin

[dblock]

See https://github.com/pixelslip/Colorkit.

https://github.com/pixelslip/Colorkit/blob/1d95e94ad45cfa842317df850c1b9abc913cc270/Colorkit.m

[dblock]

pixelslip/Colorkit#1

[thisandagain]

Major 👍 on this... I love the interface for this in LESS as an additional point of reference:
http://lesscss.org/functions/#color-operations