Add Github Pages deployment to CI

[subdavis]

I think it would be nice if we could deploy the example app to GH pages so that other folks in D&A can poke around and give feedback/suggestions. It's also generally good practice for communication.

This should be a pretty easy addition. I'm happy to do it if this sounds valuable.

[jcfr]

Here is an example I worked on. Note that within CircleCI we can also distinguish between release deployment and regular build.

• See deploy_website_command in https://github.com/amueller/word_cloud/blob/b79b3d69a65643dbd421a027e66760a4398e91b3/.circleci/config.yml#L39-L44
• https://github.com/amueller/word_cloud/blob/master/doc/deploy-website.sh