Allow loading of bmp favicons + fixes for some 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 ("//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.