Support extensions in stack.yaml

If you have multiple projects in a single stack.yaml, and want them to share the same list of default extensions, that's quite hard to do. You can specify:

```
ghc-options:
"*": -XFoo
```

But that has two problems:

* It gets applied to packages you install as dependencies that are not in your project.
* It doesn't get applied when running `stack haddock`, so the extensions aren't reflected.

One solution would be adding an explicit `extensions` key which is plumbed through everywhere.