Add wavecolour

[roaldarbol]

Hi Emil,
I've just finished a palette for wavelengths, wavecolour - the package also contains some functions for translating wavelengths into RGB and hex codes. Would be cool if you could add it to the list!
It is opinionated, and should only really be used for visualising visible light/wavelengths, I've tried to stress that point in the README. Thanks!
Mikkel