Support external links in PostList

[choldgraf]

Description

It would be useful if you could include external links in a PostList. This is common for blogs that wish to keep lists of affiliate links (e.g. to medium posts etc) in addition to local content.

I think that it could work through some combination of:

• In a post, allow for an external_link parameter.
• Somewhere around here check for the external link parameter, and if it exists, simply create an external ref that points to the link rather than an internal ref.

Then the user workflow would be:

• Publish a blog post externally
• Create a page in your Sphinx site, use ablog directive or frontmatter to add external_link param.
• All other configuration of the page relative to other posts would be the same

If folks think this would be useful, and nobody thinks I'm missing some complexity that would make this harder than it looks, I could give it a shot!