Add BNFC to LTS

[andreasabel]

BNFC-2.8.4 is on nightly, I would like to have it added to LTS.
(N.B.: BNFC is mostly an executable and does not strictly follow the package version policy.)

Checklist for adding a package:

• On your machine, in a new directory, you have successfully run the following set of commands (replacing $package and $version with the name and version of the package you want to add to Stackage LTS):

  stack unpack $package-$version
  cd $package-$version
  stack init --resolver lts
  stack --resolver lts build --haddock --test --bench --no-run-benchmarks
  

• You're using the latest version of Stack. (Otherwise please try running stack upgrade and re-running your commands. If that fails, please include the output of stack --version in this issue.)

[mihaimaruseac]

Added to LTS-16.23

[andreasabel]

Great, thanks!