Allow loading of bmp favicons + fixes for other favicon fetching-related things

Commit 1 adds BMP files to the list of loadable favicon image types.

Commit 2 changes the parsing of \<link rel="shortcut icon" ...\> tags to handle protocol-relative URLs (e.g. href="//abs.twimg.com/favicons/favicon.ico") and absolute URLs.
I also changed the regex in the parseShortcutIcon function because it didn't match correctly when there were multiple \<link\> tags in the html.