External Link icon

Added external icon to external links in _includes/_navigation.html. Based on condition `{% if link.external %}` which is already in use in the file.

I prefer to see the distinction of the external links. You can see it in action at [http://charliegriefer.github.io/](http://charliegriefer.github.io/) (note that the RSS icon you see at that site is _not_ part of this pull request). I get that this is personal preference, and YMMV. Your call, obviously.

As an alternative, if you're not a fan of the icon always automatically showing for external links, I could add a new variable to the _config.yml that would determine whether or not to display the link. `link.display_ext_icon` or some such.

Also, I've never worked with icomoon before. Please feel free to verify that I didn't muck up any of the existing icons :)