Add postall command and support for rss content

[LukeLR]

This allows the user to immediately read previous content from the feed in Matrix. Also useful for debugging purposes, not having to wait until the next post on the feed to see if it works.

[LukeLR]

This now also adds an $content template variable that contains the content of the RSS feed entries. Useful for feeds that don't have a meaningful $summary.

[LukeLR]

This now also supports regular expressions inside the template string using {{ }} double braces. See README.md for an example.