Infer missing date

This PR adds an option, `INFER_MISSING_DATE`, to `conf.py`.  If enabled, a post doesn't have to have an explicitly defined date.  Instead, the most recent git commit date of that post will be used as the date; if there is none, then the post will fall back to the creation date for the file.

This is useful in cases where the Nikola user doesn't have access to `new_post`.  For example, if you were using GitHub as your primary post creation/editing tool, then it is very convenient to work from git commit dates instead of having to copy/paste and update the date metadata each time.