I'd like to use Markdown for documents to be exchanged with a colleague of mine. The ideal workflow would be IMHO editing the source in any plaintext editor while simultaneously viewing it in a browser. The client viewer should be able to redraw the text after each save automatically, and ideally even always switch to the most recent source file (so I don't need to navigate there manually).

It'd be nice if I could (was allowed) modify the viewer a bit, things like using trailing spaces for line breaks are really terrible (I don't see them, my editor strips them, git complains about them, etc.).

I'm interested in a software capable of this and easy to modify, and also in your opinions on the described workflow.