Support HTML start tags with new lines in them.

HTML5 Start tags can contain "one or more space characters" (8.1.2.1 #3).

A "space character" is space, tab, LF, FF or CR (2.4.1).

This change makes marked support any Unicode white space in tags by restricting
the autolink to not contain any /\s/ character. It is slightly more permissive
than the HTML5 standard, but that is unlikely to be a problem in practice.