Add the ability to add links to external blog posts within my blog feed.
This is done based on a new `field_external_link` field that allows for
adding the external link URL and the domain name as the title.
The node links are then overridden to use the external link if there is
one, so the node title and 'read more' links are changed to use the
external link.
Currently, automated tweets are not generated for external posts.
Fixes#182
