[RFE] 'subfeature'

[jaymzh]

subfeature would, by default make a feature branch off the current branch instead of origin/<most_main> (or another branch if passed in)

The most basic version of this feature is just to get feature_prefix for making branches based on something other than <most_main>.

But ostensibly we could take this further, and maybe make sj up smart in this case (if the base branch has gone away, move it up to <most_main>, otherwise rebase on top of <parent_branch>