Submit independent PRs from the same stack

[jdai8]

I often find myself working on multiple unrelated changes that can be merged independently. I'd like to do all this work from a single stack to avoid updating my working copy every time I want to work on a new change (which can be expensive and invalidate build caches).

#239 suggests using histedit to re-order the stack if I want to land a later PR first. However, this is pretty clumsy if you have multiple PRs which could be merged concurrently.

Ideally, I'd want a "cherry-pick" option on sl pr submit (inspired by a similar spr feature) that cherry-picks the change onto master at submit time. The submitted PR would target master so it can be merged as normal on Github. Is it possible to do something like this?