Add highlight for search results

Looks like it might be enabled via URL query parameter: 
https://sphinx-rtd-theme.readthedocs.io/en/stable/index.html?highlight=sphinx

<img width="1104" alt="image" src="https://user-images.githubusercontent.com/4189262/70798164-23bf4f80-1d9e-11ea-872f-60d9cb9ee542.png">
