Allow more fined grain control when pushing or submitting branches

Often my stack is ordered by quality or experimental-ness, so commits close to the bottom are core dependencies that are most likely going to be committed, while the top of the stack is chaos.

When calling `gh stack push` or `gh stack submit`, it would be helpful to be able to push or submit just a portion of the stack. Right now, I am dropping down to git and pushing manually.

Maybe just support `--upstack` and `--downstack` for those commands as well.
`gh stack push --downstack` push only from truck to current branch.