option to ignore entities processing

hi.

I need this for [svgo](https://github.com/svg/svgo) because:

1. I need to serialize XML (SVG) parsed to AST back into a string that might be embebbed inline to HTML, so I need to preserve all the original entities "escaping"
2. entities declarations from doctype section should "works", i.e. just be ignored without `Invalid character entity` error
3. strict mode should still be turned on

so.. I just need to disable entities processing to keep text nodes and attrs values in the original state.