Resolve links to local Markdown file to their HTML counterparts.

I have added a small feature to ensure generated HTML files preserve links to other local HTML files if the original Markdown files have been linked (-r option). Also added a command line option to inject a CSS file into generated HTML to allow styling of the generated HTML (-c Markdown.css option).

I had to implement a couple of fixes in the Markdown library used by project. The bug have been reported to the original project, hopefully they will release the latest NuGet soon, at which point the fix can be removed.