quickly view markdown presentation file using notes-server

Hi,

This adds the ability to quickly view an external markdown presentation file via ```nodejs plugin/notes-server/ [markdown_path]```.

It's still rough, needing a doc update and such, but I want to see if it is something you are interested in. If so I am considering separating the current index.html into a template file containing the common content and a second file containing the demo presentation. Grunt can then build the combined index.html as well as the new mdindex.html without needing to keep duplicate content around. I am also considering making some or all of the options available as arguments though, in which case I suppose there would be less duplicated.