* Add functions for setting description and OG meta
It's common that a website author will want to add a specific
description and Open Graph image to a given page in their website. These
functions are simple conveniences to add these meta tags to the document
head.
I decided against simply adding all possible meta tags, because not all
of them are useful, and even in the case of Open Graph tags, many of
them should be set only once and they should use the same value for the
entire website. In those cases, it's probably better for the website
author to add those tags in their layout template.
Closes https://github.com/yesodweb/yesod/issues/1659
