Add Rec2020 color space

[tomcur]

Resolves #20, now implementing all CSS Color Module Level 4 color spaces.

Further:

• Add Tom Churchman to AUTHORS list.

[raphlinus]

Oh, also please add your name to the AUTHORS list.

[raphlinus]

Btw, we can now delete the allow for todos. That could have been done in this PR, but I'll do it in an upcoming one that improves docstrings.

[DJMcNab]

Fwiw, that's another key advantage for expect - the self-cleansing aspect.