improve style of Parser#favicon
handle additional favicon markup scenarios

    <link rel="icon" href="/src/favicon.ico">

or

    <link rel="shortcut icon" href="/src/favicon.ico">

or

    <link rel="shortcut" href="/src/favicon.ico">

or

    <link href="img/apple-touch-icon-58x58.png" rel="apple-touch-icon" sizes="58x58" />
    <link rel="shortcut icon" href="/src/favicon.ico">
Merge pull request #79 from jjb/favicon-shortcut

Favicon shortcut