Limit RSS feed contents and remove headerlinks

Add a new option, rss_max_items, to control the number of posts that
appear in RSS feeds.

Make the code for building the feed context more generic, based on the
work in tinkerer-contrib/rss/rss.py.

Strip anchor tags with the class headerlink from RSS item descriptions,
so when the links are used in a site they do not appear in the RSS
output where they don't make sense as permalinks.

Change-Id: Ia90d70389152a9fda1137606eb6eef9918cbc221
Add --template command line option

The --template (-t) option allows the user to specify
a template for their input other than the single default
template used for pages and posts. This is useful for
cases where many posts with similar contents such as
package release announcements.

Change-Id: Id0ab6dea70983cd8f22235858515db91119ca728
Merge branch 'clean-rss' into doughellmann.com
Merge branch 'content-templates' into doughellmann.com