Add support for favicons

[seshrs]

Context

EECS 485 has a custom favicon, and other courses' staff may be considering adding favicons to help students find the project-spec tabs in their browsers. This PR allows course staff to configure favicons to be used for all pages on their site.

I also think it would be great to see this feature make its way to the original Primer theme. I've opened pages-themes/primer#42 to request adding this support.

Validation

Pull the branch locally.

1. Add the following line to _config.yml:

   favicon: https://eecs485staff.github.io/eecs485.org/favicon.ico

   Run script/server and visit http://localhost:4000 —- the site's favicon should display in the browser tab list.

   

2. Remove the line from _config.yml and restart script/server. Refresh the page on your browser, and notice that the browser uses a default placeholder favicon in the tab list.