Add a period archives feature and brief docs.

Allows users to have per-year, per-month, and per-day archives of posts
automatically generated. The feature is disabled by default; to enable
it a user must supply format strings for a period's respective
`_SAVE_AS` setting.
documentation for IGNORE_FILES
Added missing JINJA_FILTERS documentation
Added missing SITESUBTITLE documentation
Fix typos
Merge pull request #779 from johnmastro/add-period-archives

Add period archives
Merge pull request #785 from wraithan/docs_for_ignore_files

documentation for IGNORE_FILES
Fix typos
Merge pull request #786 from noirbizarre/more-documentation

Add missing documentation for JINJA_FILTERS and SITESUBTITLE