Add optional tag-specific feeds

This is useful for posting to 'planet' like feed aggregators, when an
author would like to have only a certain subset of posts aggregated.

For example, if a blog contains a mixture of Eclipse-related content,
and pictures of cats wearing silly hats; then only the posts tagged
with 'eclipse' should appear on http://planeteclipse.org/planet/

This requires a new template file. I've created ones for the
Freemarker ([here](https://github.com/jbake-org/jbake-example-project-freemarker/pull/10)) and Groovy ([here](https://github.com/jbake-org/jbake-example-project-groovy/pull/1)) example projects.