Auto-generate require files for mu-plugins

[simonwheatley]

Whenever Composer installs a package as a mu-plugin, i.e. a directory in wp-content/mu-plugins, it generates a plugin file in the wp-content/mu-plugins directory which requires the MU plugin… so you don't have to.

Things to consider:

• Create/update a require file when a package installs/updates
• Remove the require file when a package is uninstalled
• Move the script to it's own repository/package