Bring things under 80 columns where possible
Fold colour scheme set up into niji#highlight()

The colour scheme that niji#highlight() uses is based upon :colorscheme
or &background. Currently this is chosen for one invocation of
niji#highlight(). Now, this will be chosen upon all invocations, to
support changing :colorscheme or &background.
Call s:load() upon ColorScheme

Automatically update parentheses highlighting on changes to colorscheme.