Add more robust support for HTML5 anchor tags.

[xStrom]

This PR makes the anchor tag regex more robust so it can handle more of the HTML5 spec.

Old   a.*href=('([^']*?)'|"([^"]*?)")
New   ^(?i:a)(?:$|\s).*(?i:href)\s*=\s*('([^']*?)'|"([^"]*?)"|([^\s"'`=<>]+))

To break down the changes, in order:

Regex	Reasoning
^	Check that the tag actually starts with the letter a, as opposed to say in the case of head.
(?i:a)	Tags and attribute names are case insensitive, so we need a case-insensetive check.
(?:$|\s)	We want only the letter a and not say article, so we check for the end the same way as badTagnamesRE.
(?i:href)	Case-insensetive check for attribute names as well.
\s*=\s*	The equals sign can be surrounded by zero or more spaces.
|([^\s"'`=<>]+)	Attribute values don't have to be enclosed in quotes if they follow certain rules.

[k3a]

Thanks for the nice PR! I will make other tag regexpes case-insensitive as well in the followup commits.