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

Reproduce the report by running hlint . on the referenced README.lhs with a current HLint version. Start by tracing how literate Haskell and its preprocessor directives are handled, then establish the expected behavior for the reported directives. Done means the README.lhs input is processed without the listed unknown-directive warnings.