Create Markdown configuration

[kostiantxn]

We should create a convenient configuration mechanism for configuring the pipeline.

As @joshua-light mentioned in #46 (comment):

What do you think about extracting a new MdRendererOptions class for tweaking such things?

For example, we could call the UseMdRenderer method on the pipeline with few ways:

(the old one)

pipeline
    .UseMdRenderer("...");

(the more customizable one)

pipeline
  .UseMdRenderer(options => options
    .CleanupOutputDir()
    .UseGitHubMarkdown())