Local images and navigation

Thanks for a great plugin! Just wanted to share some of the changes I have made. I forked the repo a couple of days back and now, as I come back few days later, l find [pull request from metacpp](https://github.com/NoahRic/MarkdownMode/pull/18). Looks like metacpp is trying to address the same problems as I am. I have also added support for local/relative images to display in the preview window. Our solutions look very similar, however based on your comment from the link above, looks like you are still trying to make a few changes to their pull request. Have a look, may be mine is a bit closer to what you wanted.

In addition to local image display I have added support for navigation to documents within current solution. This allows links to source files or other Markdown documents and creates site (wiki) like experience by enabling user navigation between Markdown files.

I followed your coding style as closely as possible to keep everything consistent, however please let me know if you want me to improve anything.

__NOTE:__ All the changes have been made and tested using VS2013. To do the changes I had to upgrade the solution. This pull request does __not__ include upgraded solution and project files. I have gone over the code and it looks like it should still work in VS2010, however I apologise in advance if this breaks the build.