Add a develop option

[Julian]

Distinct from #3, which asks for installing packages -e, this option (or some even better name) should make it so that you can do:

[virtualenv.foo]
install = ["foo"]

[virtualenv.foo.develop]
python = "python3.7"
install = ["$DEVELOPMENT/foo"]

or something, and the second venv would get all of what's present in the parent venv, but replace foo with an editable install of it, for when you want to use and develop a package at the same time.