Add Colour class (equivalent to Color)

[ahmadSibai]

Since the package name is colour (with 'u'), having a class name 'color' is a bit imperfect, thus having a 'colour' class would nail it.

[codecov[bot]]

Codecov Report

Merging #46 into master will increase coverage by <.01%. The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #46      +/-   ##
==========================================
+ Coverage   99.13%   99.14%   +<.01%     
==========================================
  Files           1        1              
  Lines         232      233       +1     
==========================================
+ Hits          230      231       +1     
  Misses          2        2

Impacted Files	Coverage Δ
colour.py	99.14% <100%> (ø)	:arrow_up:

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