define variablese
temporarily alphabetize color vars, add commercial
use color variables in landcover.mss
Merge branch 'color-variables' of https://github.com/ian29/openstreetmap-carto into ian29-color-variables
Fix typos in comment, colour and polygon-fill.
Move landcover colour variables to landcover.mss

This can be revisited in future, but it keeps it in line with what's
happening in roads.mss
Merge branch 'ian29-color-variables'

Closes #124