* guix-kernel/magic.scm (magic-html?): New procedure.
* guix-jupyter-container.scm (reply-html-to-kernel,
  kernel-info->html): New procedures.
* guix-jupyter-kernel.scm: Minor change.
* guix-kernel/html.scm: New file.
* guix-kernel/jupyter-client.scm (reply-html): New procedure.
* Makefile.am: Add 'guix-kernel/html.scm'.
* guix-kernel-demo.ipynb: Add example for new features.