Add a `develop` option

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

```toml
[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.

Start by reading the existing configuration handling for the [virtualenv.foo] entries described in the issue and trace how environments and package installs are created. Done means a named develop option creates a second environment from the parent while replacing the selected package with an editable install, with tests covering the configuration and resulting environments.

Written by the indexing model from the issue text.