Allow extending article footer and header

[leonidasv]

The extend-article-link, extend-head and extend-footer partials are nice, but I felt the necessity of extending the article's footer when I was developing a post suggestion engine (I plan to open-source it soon!).

The straightforward proposal would be to add an extend-article-footer partial, but I want to propose something else: why don't we move the <footer /> code at the end of _default/single.html to a separated single-footer.html partial that _default/single.html can use?

This way, I could override just the article footer partial and put my code there (in fact, for my usecase, it's better than extend-..., because I want my recommendations to appear before the author section. It's more flexible than a extend partial). We could also do the same for the article heading.

Rationale: it's easier to maintain an override of just the footer than to maintain an override of the whole single.html file.

What do you think?

(Preview of the proposed changes: dev...leonidasv:congo:leonidasv/single-page-footer-refactor)