Add period to period_archives context. Refs #1044.

* Adds period tuple of (year, month, day) matching the time
  period of the current archive. Note that this is done
  to the archive context if period_archives.html doesn't exist.
* Adds tests to verify this.
* Adds documentation in themes.rst about period in period_archives.html.
Merge pull request #1246 from fenekku/period-to-archive-context

Add period to period_archives context. Refs #1044.