Favicon: update plugin element selector

This change addresses the W3C no longer allowing link[rel="shortcut icon"]
for favicons.  Plugin now looks for link[rel="icon"] elements to determine
if a mobile favicon should be added.
Favicon: update working examples with link[rel='icon']
Favicon: update docs with link[rel='icon'] change
Merge pull request #5076 from patheard/favicon

Favicon change to rel="[icon]"