Optional favicon checker

Absent favicons cause 404s, because browsers expect at least one favicon. Examples for @parkr’s website:

* [In Chrome](http://www.webpagetest.org/result/140719_80_81G/1/details/)
* [In Firefox](http://www.webpagetest.org/result/140719_NR_82B/1/details/)

One way to embed favicons is to place a `favicon.ico` file in the root folder. This is not checkable.

My addition checks the other way whether a favicon is embedded with a link element.

* [x] Implementation
* [x] Documentation