Improve Post Format Archive Headings.

Follows up on #95 and https://github.com/Automattic/_s/commit/e3b4907946696c324877b775bdfea8314af1799a#diff-ff638db82c6ac72a05781ba3e086fdf0L48.

The comment made on the original commit is as follows:

> It's worth noting that post formats not accounted for (i.e. not added into wp-admin) should maybe also be added into this template file. What I mean is that if you are using a post with format status and then switch to an _s-based theme, the post will still have the post format status and you will still be able to view it by going to /type/status. Maybe we should take all post formats into account even if support for them isn't explicitly added into functions.php.