Make --hyperlinked-source for haddock optional

[Fuuzetsu]

General summary/comments (optional)

I would like to be able to tell stack to not give --hyperlinked-source to haddock with stack --haddock. Currently it always invokes this flag if it's supported.

Steps to reproduce

1. stack --haddock -v

Expected

Configurable option.

Actual

Option always passed in without means to turn it off.

Stack version

1.4.0, checked source and no reason to believe it's any different in git master

Method of installation

N/A

[mgsloan]

Not sure why you'd want to turn this off, but sure, being able to disable it makes sense.

[Fuuzetsu]

In my particular case it's to workaround a Haddock bug. In general though we may want to host documentation and show it around without exposing source (for whatever reason).