Add support for `on_plugin_install` in plugins

 - Add support for `hooks/{Windows,Linux,Darwin}` directory in plugins.
 - The _(optional)_ `hooks/.../on_plugin_install.{sh,bat}` script in a
   plugin is run when a plugin is installed.
 - The _(optional)_ `hooks/.../on_plugin_uninstall.{sh,bat}` script in a
   plugin is run when a plugin is uninstalled.
 - The path of the Python executable used to run MicroDrop is passed as
   the first argument to hook scripts.  This helps to run, _e.g.,_ `pip`
   installations within the appropriate Python installation.