Do not show GitHub button if no github_user or github_repo specified
Do not show GitHub button if no github_user or github_repo specified

Fixes #51.

jupytercalpoly/jupyter-alabaster-theme,ellisonbg/jupyter-alabaster-theme,ellisonbg/jupyter-alabaster-theme,charnpreetsingh/jupyter-alabaster-theme,charnpreetsingh/jupyter-alabaster-theme,ellisonbg/jupyter-alabaster-theme,charnpreetsingh/jupyter-alabaster-theme,jupytercalpoly/jupyter-alabaster-theme,jupytercalpoly/jupyter-alabaster-theme