Support for double-quoted Heredoc identifiers

Introduced with PHP 5.3.0, as described in
http://php.net/manual/en/language.types.string.php#example-90

This syntax is *not* (yet?) in the php spec (https://github.com/php/php-langspec/blob/master/spec/09-lexical-structure.md#heredoc-string-literals) - but I'm trying to get this parser to eat some code which uses it :)