- Use `BookLab::HTML` instead of `BookLab::Markdown` and support `html`, `sml` and `markdown`;
- Add full feature of `SML` format support;
- Add `format` field to `docs`, `versions`, and render html by `format`; 
- Avoid use `markdown` helper in model, use `BookLab::HTML.render` instead.
- Now `BookLab::HTML.render` will including caching and sanitize html;
- Add more attributes support for `BookLab::Sanitize`;