Added a derived class for Markdown to work around the implementation bugs.

A pending pull request (https://github.com/toptensoftware/markdowndeep/pull/43)
will resolve the issue. Once a new NuGet of the lib is released this class can be removed.
Added support to resolve links to markdown files to their HTML counterparts.

If the source Markdown files contained links to other local Markdown files the
links are resolved to HTML files that are generated in their stead. If the source
Markdown files were linked generated HTML files will be as well.
Merge pull request #1 from bigT/feature-local-md-resolution-and-css

Resolve links to local Markdown file to their HTML counterparts.