This adds lexing support for HTML/XML such as:
<foo bar="""></foo>
While technically invalid, some websites (e.g. yahoo.com) contain HTML
just like this.
The lexer handles this as following:
1. When we're in the "element_head" machine, do business as usual until
we bump into a "=".
2. Call (using Ragel's "fcall") the machine to use for processing the
attribute value (if any).
3. In this machine quoted strings are processed. The moment a string has
been processed the lexer jumps right back in to the "element_head"
machine. This ensures that any stray quotes are ignored instead of
being processed as extra attribute values (eventually leading to
parsing errors due to unbalanced quotes).
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