Add "blend" subcommand

[sharkdp]

In analogy to "mix".

Resources:

• https://www.w3.org/TR/compositing/#blending
• https://en.wikipedia.org/wiki/Blend_modes
• http://sharkdp.github.io/purescript-colors/#blending

Make sure that we can reproduce this: https://stackoverflow.com/questions/13669491/are-there-ways-to-use-additive-color-mixing-in-web-development