Warn when colour too light / too dark

[lah7]

it would be beneficial to show a warning when a colour is out of a specified hex range. Plus, a command parameter (--ignore-brightness could ignore this warning and proceed anyway.

The warning could say:

Colour too light, try a darker colour or use --ignore-brightness

Colour too dark, try a lighter colour or use --ignore-brightness

This is so custom generated colours do not inhibit "black spots" due to being too dark and vice versa with too light colours.

[Martinique]

This seems somewhat related to #24. With the luminosity method (a weighted RGB to grayscale conversion) it's possible to calculate if a color has too little contrast with black or white. Then it's just a matter of choosing threshold values for the warning.