Exit if accessed directly
Remove Github URI reference

`GitHub Plugin URI:` is used by [Github Updater][gh-updater] plugin
to provide automatic updates to Github hosted plugins.

While this project is hosted there, we do not want to require a second
plugin to make updates possible. A future version should include a
handler for updating and be kept internal to the plugin.

[gh-updater]: https://github.com/afragen/github-updater