Support configurable base branch for stack operations

[trotterdylan]

Currently all of the plz stack operations assume main is the base branch (actually, the "default" branch configured in GitHub). We should make this configurable so that other branches can act as the base, e.g. release or deploy branches.

The simplest approach would be to support a --base-branch option for the relevant commands, but that can be onerous, so supporting a persistent configuration option may be valuable. Per a user suggestion:

something similar to git’s config, and use:

plz config --global plz.base develop
plz config plz.base develop

which would set that config setting in ~/.gitconfig or .git/config if it were global or not, with the —base=develop being overridable on the command line.