Support for extending markdown interpreter

Expected Behavior

I'd like to be able to extend Remarkable with custom configuration/plugins (https://github.com/jonschlinkert/remarkable/blob/master/docs/plugins.md).

Current Behavior

Remarkable is completely instantiated and consumed within the context of the Markdown provider, without passing in any configuration sources.

Possible Solution

Allow configuration options to be passed to Remarkable via a property of the Swagger UI configuration object.

Context

I'm attempting to use a plugin to render specific code-fenced blocks using an external library, instead of rendering them as per standard. As they are rendered within the Markdown flow (and rendered to escaped HTML before I can intercept), I am unable to do so.

Start with the Markdown provider, where Remarkable is instantiated and consumed, then review the Swagger UI configuration object and Remarkable's plugin documentation. Done means configuration can pass Remarkable options or plugins through Swagger UI so custom rendering of fenced code blocks occurs before the output is escaped.