Use RSS feed template for mail notes

[MelissaAutumn]

Requires #29

We should add an option for preview.py to generate the notes using the rss feed template: https://github.com/thunderbird/thunderbird-website/blob/master/sites/www.thunderbird.net/includes/release-notes-feed.html

We can then adjust this script's subprocess call to use our new flag
https://github.com/thunderbird/thunderbird-notes/blob/master/tools/mk_release_announce_email.py

The feed template produces simple output which is more easily transferable over email. We may need to make some tweaks though.

Example/