Fix prune scoping issues.
Change branches_cmd from a function to a lambda.

This will allows branches_cmd to use the parent scope for stable_branch
and development_branch.
Change all functions to lambdas in feature prune.

This is done simply to pass in less parameters and, thus, clean up the
code.
Merge fix-feature-prune (#104) into master

Fix feature prune since stable/dev branches were unhardcoded.

The stable and dev branches now need to be passed in since they are no
longer hardcoded.