Have setup-info support specification of a ghc build directory

I haven't put a great deal of thought into this idea, but it seems like it's a good feature for those developing on GHC while using stack. The idea is to allow pointing at the root of a GHC source tree, or its `inplace` dir, and running it directly out of there.

So, in your global `config.yaml`, you could have something like the following:

```
ghc:
linux64:
8.0
version: 8.0.2
ghc-source-path: /home/mgsloan/oss/haskell/ghc
```

And it would magically use your development version of GHC!

I hope to be returning to iterating on my in-flight ghc patch soon, so that will be a good excuse to revisit this.