Match HTML DOCTYPE insensitive of case

This allows for case insensitive matches of the start of the HTML DOCTYPE.

From the HTML 5 spec ([link](http://www.w3.org/TR/2014/CR-html5-20140731/syntax.html#the-doctype)):

> A DOCTYPE must consist of the following components, in this order:
>
> 1. A string that is an ASCII case-insensitive match for the string "`<!DOCTYPE`".

The closest thing I could find to a syntax for the HTML 4 DOCTYPE says ([link](http://www.w3.org/TR/html-markup/syntax.html#doctype-syntax)):

> A deprecated doctype consists of the following parts, in exactly the following order:
>
> 1. Any case-insensitive match for the string "`<!DOCTYPE`".

> A legacy-tool-compatible doctype consists of the following parts, in exactly the following order:
>
> 1. Any case-insensitive match for the string "`<!DOCTYPE`".