support --ignore-link-symbol in OPTIONS_HADDOCK

[sellout]

At work we fail the build on Haddock warnings. We use a global list of symbols to pass to Haddock via --ignore-link-symbol, but it would be very helpful if there were a way to say that we want to ignore-link-symbol in the module that defines the symbol that is intentionally not exported.

The most obvious way to do this is to allow something like {-# OPTIONS_HADDOCK ignore-link-symbols=A.B,A.C #-} (although maybe there are others). It would also be nice if the symbols didn't have to be qualified (if they're not, they're assumed to be from the current module.