Consider a way to make the extension configs extendable

[ehuss]

Currently mdbook allows preprocessors and renderers to define their own config settings in their corresponding configuration table. For example, mdbook-linkcheck has a bunch of settings. However, these settings share the same namespace with mdbook itself which has settings like command, optional, renderers, before, and after. Each time mdbook adds its own configuration setting, there is a risk that it can conflict with existing extensions.

I think it would be good to consider a different structure to allow mdbook to add new settings without potentially interfering with existing extensions. This would allow new settings to be added in a backwards-compatible way.

I don't really know what that would look like. It could be some different sub-table like output.name.settings?