Better favicon support

[alanorth]

Right now, with zero other configuration, you can put a favicon.ico in your site's static directory and basically any browser will find it. There are better, more modern ways to do favicons now, though, for example:

• support for high res icons
• support for specific device features like iOS, Android, and Windows

Generating the code/images for this is easy enough (see realfavicongenerator.net) but it would be nice to have a configuration flag for controlling the addition of this code.

To play devil's advocate, though, it might be better to just advocate adding this site-specific code to head-custom.html (see #17). Keeping the theme and its configuration simple is a plus.

[alanorth]

Actually, the head-custom.html method is sufficient, and gives the site owner complete control over the favicon.

See alanorth/picturingjordan.com@6994e96 for a very quick example generated by realfavicongenerator.net.