Add haddock one-shot mode

One-shot mode for haddock is meant to be used with build systems like Bazel or Buck. The idea is to be able to incrementally build the documentation one module at a time, while being guaranteed that no recompilation is happening.

There are two big downsides to this approach:

1. It is fundamentally incompatible with showing all instances for a class.
2. I was not able to get the right DynFlags from a .hi file. If there's a .hie file we can use getOptions to reparse them from the source file, but that's a nasty workaround.