If no mention source image, use default.png
Minor improvement

locally cached image (in mentions) extension will be stripped
prom any parameters. For image.jpg?version=123 the extension
is now '.jpg' (was .jpg?version=23)
Extract all links from rendered post

All links included in a post will be extracted by render_post()
and appended to .bucket3/extarnal_links.txt

This file is intended to be used by other tools to send
pingbacks and webmentions.
All fixes and new features will be part of v0.12.0