Add options to include portions of files

[djboulia]

I use markdown for the lecture portion of a class I teach. There are lots of embedded code examples. The samples are usually part of a larger code module, so it's convenient to be able to just include a few lines of the source in the markdown. I also often jump between the markdown lecture material and the actual code, so having a hyperlink to the complete source file is also useful. This PR adds options to the existing include to allow specifying a start and end line for including a subsection of the file. It also adds a link and vscode option. link will just provide a hyperlink to the original include file. vscode will include a link that will open Visual Studio Code with the given file and starting line.