html.lang: fix regression in quoted attribute context

The \" (string beginning) must be at the first <start>, otherwise it can
match text like:

<a href=blah"foobar">link</a>

And the 'end' sub-pattern is no longer needed.
html.lang: add embedded-lang-hook in unquoted attribute

https://bugzilla.gnome.org/show_bug.cgi?id=564345
html.lang: improve style-ref for attributes

The attrib-name style was applied to the entire context. It is now
applied only to the attribute name (plus the = sign).