[ENH]: Ability to add custom colours and colourmaps to matplotlibrc

[tom-new]

Problem

I have a custom colour map that I define with LinearSegmentedColormap that I use a lot in my work. It has 17 different colours and therefore clutters up the top of all my plotting scripts (and it's annoying having to copy it every time I want to use it).

Proposed solution

It would be convenient if custom colours and colourmaps could be somehow defined in the matplotibrc file, so we can access them without having to define them every time. If there is already some way to do this globally, that would be great, but I couldn't find such a way (aside from editing the matplotlib source files, which doesn't seem like a good solution).

[story645]

Are you packaging your custom colormaps, like for example https://github.com/matplotlib/cmocean