Rufflewind authored Nov 29, 2016 and Ben Gamari committed Nov 29, 2016

This is a preliminary commit to add colors to diagnostics (warning and
error messages).  The aesthetic changes are:

  - 'warning', 'error', and 'fatal' are all colored magenta, red, and
    red respectively.
  - The warning annotation [-Wsomething] shares the same color.
  - Warnings and errors are also bolded (this is consistent with what
    other compilers do).

A new flag has been added to control the behavior:

    -fdiagnostics-color=(always|auto|never)

This flag is 'auto' by default.  However, auto-detection is not
implemented yet, so it effectively it defaults to off.

Test Plan: validate

Reviewers: austin, bgamari

Reviewed By: bgamari

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D2716

GHC Trac Issues: #8809