Add "exclude" (and/or "include") option to parser options

I'm working on a chat application, and want to offer a (quite conservative) subset of markdown, but it turns out to be next to impossible to find any way to do so without writing something from scratch.

It would be really great if it were possible to pass in a set of desired formats (and/or a set to exclude), and have the parser only parse the resulting set of formats.

For example, we would like to only support code_fences, code_spans, bold, and italic. (think slack, but commonmark-compliant).

From checking the implementation, it seems to me that checking the options at construction and removing the functions for undesired formats from the parser instance would be a simple enough way to provide this functionality.

I'd be willing to PR this myself if the feature is accepted.

Start by tracing parser construction and how parser options are handled. Define how include and exclude sets interact, then verify that only the requested formats—such as code fences, code spans, bold, and italic—are parsed while other formats remain unsupported.