Feature Request: init based on nested stack.yaml

### General summary/comments (optional)
If feasible, perhaps consider creating a variant of of `stack init` that utilizes nested `stack.yaml`'s instead of nested `cabal`'s

### Example
Currently
````
mkdir newdir && cd newdir
git clone https://github.com/gibiansky/IHaskell.git
git clone https://github.com/llvm-hs/llvm-hs.git
stack init
````
finds about ~16 `cabal`'s, and then goes about doing its thing trying to find a compatible snapshot.

------

It would be nice to be able to do:
````
git clone https://github.com/gibiansky/IHaskell.git
git clone https://github.com/llvm-hs/llvm-hs.git
stack init-from-substacks --depth 1
````
which would find ~15 `cabal`'s, ignoring 1 problematic `cabal` that is unreachable from the `depth=1` `stack.yaml`'s, and also add
````
flags:
llvm-hs:
shared-llvm: true
````
to the newly provision `stack.yaml`. I'm somewhat new to all this, apologies if this utterly infeasible, silly, or rtfm.
cheers, nehal