Separate haddock doc URI from documentation path

Hi,

I'd like to be able to expose haddock documentation over HTTP so that, for instance, I can develop on one machine and browse the docs for the library I'm working on from another, or so that we can keep a copy of the docs for a particular build.

This _almost_ works already, except that the `haddock-html` field in the package database always points to the filesystem path at which the docs are installed (as set by `--htmldir`), which means that cross-refs between packages don't work when visited over HTTP.

Would it be reasonable to add another setting that allows the `haddock-html` field to be set to something other than a filesystem path? I can attempt the changes myself if you think this is a good idea to try.

Many thanks,

David