Start using variables for colors

[ian29]

I've only started on this for the colors present in landcovers.mss, but it will unlock tasks like #56 and #57 (or at least make them much much easier to implement)

I also recognize that this is somewhat against the spirit of this bit from the README:

Care will be taken to not get too clever with variables and expressions. While these often make it easier to customise, experience has shown that over-cleverness (e.g. interpolated entities) can discourage contributions.

While I don't want to discourage contributions, I think making the style more structured + logical is important. Keeping consistent colors across thematic features will be important for making the map design flatter and more legible as we progress on Version 3 improvements.

All that being said, I'd love to hear reasons why this is might hurt contributions.

[gravitystorm]

Apologies for taking so long to review, update and merge this! Thanks for the work.

[ian29]

:D