Support for lexing empty attribute values.

This ensures that Oga can lex the following properly:

    <input value="" />

Previously Ragel would stop upon finding the empty string. This was caused due
to the string rules being declared as following:

    string_dquote = (dquote ^dquote+ dquote);
    string_squote = (squote ^squote+ squote);

These rules only match strings _with_ content, not without. Since Ragel stops
consuming input the moment it finds unhandled data this resulted in incorrect
tokens being emitted.