Add option to convert all relative URLs in post content to absolute

[Kronopath]

I'm aware that you folks set Atom's xml:base property to give a hint to readers as to how to interpret relative URLs. Thanks for that.

Unfortunately, some readers, in particular RSS-to-email services like follow.it and Mailchimp, apparently don't support xml:base properly. This is especially problematic when it comes to images: several RSS-to-email services show broken images when they use relative URLs for their src fields.

It would be great if there was an option to auto-convert all relative URLs in HTML attributes (like src or href) to absolute.

Other issues like #315 and #185 have brought this up before but haven't got traction, so this seems to be a recurring issue.