Allow colors to be customized

Allow customization of diagnostic colors through the GHC_COLORS
environment variable.  Some color-related code have been refactored to
PprColour to reduce the circular dependence between DynFlags,
Outputable, ErrUtils.  Some color functions that were part of Outputable
but were never used have been deleted.

Test Plan: validate

Reviewers: austin, hvr, bgamari, dfeuer

Reviewed By: bgamari, dfeuer

Subscribers: dfeuer, rwbarton, thomie, snowleopard

Differential Revision: https://phabricator.haskell.org/D3364

(cherry picked from commit adf27d61)