Support preprocessors

When building in CI hlint was unable to parse the README.lhs in this PR https://github.com/frontrowed/faktory_worker_haskell/pull/13#issuecomment-398427029

This build utilizes the fpco docker build of lts-11.9, which contains https://www.stackage.org/lts-11.9/package/hlint-2.1.5

hlint .
Warning: unknown directive #faktory\_worker\_haskell
in ./README.lhs  at line 1 col 1
Warning: unknown directive ##
in ./README.lhs  at line 34 col 1
Warning: unknown directive ##
in ./README.lhs  at line 38 col 1
Warning: unknown directive ##
in ./README.lhs  at line 43 col 1
Warning: unknown directive ###
in ./README.lhs  at line 62 col 1
Warning: unknown directive ###
in ./README.lhs  at line 77 col 1
Warning: unknown directive ###
in ./README.lhs  at line 92 col 1
Warning: unknown directive ###
in ./README.lhs  at line 107 col 1
Warning: unknown directive ##
in ./README.lhs  at line 117 col 1
Warning: unknown directive ##
in ./README.lhs  at line 157 col 1
No hints

I did not have the same issues locally with:

$ hlint --version
HLint v2.0.10, (C) Neil Mitchell 2006-2017

However when I upgraded my local hlint I did see them.

$ hlint .
Warning: unknown directive #faktory\_worker\_haskell
in ./README.lhs  at line 1 col 1
Warning: unknown directive ##
in ./README.lhs  at line 34 col 1
Warning: unknown directive ##
in ./README.lhs  at line 38 col 1
Warning: unknown directive ##
in ./README.lhs  at line 43 col 1
Warning: unknown directive ###
in ./README.lhs  at line 63 col 1
Warning: unknown directive ###
in ./README.lhs  at line 78 col 1
Warning: unknown directive ###
in ./README.lhs  at line 93 col 1
Warning: unknown directive ###
in ./README.lhs  at line 108 col 1
Warning: unknown directive ##
in ./README.lhs  at line 118 col 1
Warning: unknown directive ##
in ./README.lhs  at line 158 col 1
No hints