Optionally leave HTML entities unparsed

Fixes https://github.com/isaacs/sax-js/issues/97

Exposes the `strictxml` or `strictEntities` option to the user, which when truth-y will only parse `&amp;`, `&apos;`, `&gt;`, `&lt;`, and `&quot;` and pass HTML entities (like `&int;`) unparsed.

Happy to change the name of the option or its implementation if requested.