Markdown Viewer: Default setting removes frontmatter

[rowanc1]

In JupyterLab the default setting automatically removes the frontmatter before sending it to the renderer. We should warn about this setting.

To turn it off:

When it is enabled, the frontmatter cannot be accessed by the renderer, and we ideally don't want to take over too many responsibilities, and so are inheriting this behaviour from the default markdown viewer.

[krassowski]

FYI extensions can change settings when enabled. This is for example what jupyterlab-execute-time does to enable timing metadata in notebook (here).