Changed
- Avoid saving to global variables during parsing. There can be multiple parsers running in parallel.
- Now the `children` attribute for block elements should be a list of child elements.
- Move the HTML content of `HTMLBlock` from `children` to `body` attribute.
- Fixed some built-in extensions that modify class attributes.
- Add a helper class to create extensions, instead of using arbitrary objects to hold partial properties.
Fixed
- Call `setup_extensions()` when running `render()` method alone.