Implement postinstall #21
Description
Some packages will probably need specific things done after they're installed or uninstalled as per #20 - e.g. a payments package will need the hooks copied over from the package to the dev folder.
It's not realistic for the CLI to know how to do this for each possible package, so probably instead have a postinstall.js file in each package, which the CLI will run after installation, if it exists.
Open questions remain around whether the single file should contain separate methods for install, uninstall and upgrade; and how the script and CLI can talk to each other to e.g. resolve conflicting hooks.