nbgitpuller Link generator

A browser extension to help you generate [nbgitpuller](https://jupyterhub.github.io/nbgitpuller/) links directly from GitHub. On the GitHub page for files, folders and repositories, it adds an 'nbgitpuller' button to the browser bar. On clicking this, you can enter a JupyterHub URL and the application you want to use to open this file, folder or repository. Then you can just copy the nbgitpuller URL, and share it with your students! The JupyterHub URL and application you choose are remembered, so you do not need to enter it over and over again.