os/platform-specific fields in stack.yaml

I think it would be nice to have a way of specifying fields that are os-specific. Something like this:

```yaml
os-specific:
windows:
ghc-variant: integersimple
flags:
hashable:
integer-gmp: false
sceintific:
integer-simple: true
...
```
Thus, one can keep more build information within stack and write less additional scripts.

NOTE: This could be possible also by modifying the confHook in Setup.hs if stack didn't bypass the configure step.