Summary:
Frontend plugins enable users to write plugins to replace
GHC major modes.  E.g. instead of saying

    ghc --make A B C

a user can now say

    ghc --frontend GHC.Frontend.Shake A B C

which might provide an alternative implementation of a multi-module
build.  For more details, see the manual entry.

Signed-off-by: Edward Z. Yang <ezyang@cs.stanford.edu>

Test Plan: validate

Reviewers: simonmar, bgamari, austin, simonpj

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D1598

GHC Trac Issues: #11194