Allow customizable file extensions

[roytruelove]

A more general request than #22. With the introduction of literate programming, markdown files will not necessarily have an .md extension (eg literate coffeescript uses .litcoffee).

Ideally we could specify the extensions that this plugin should convert.

[thedaynews]

I have the opposite problem. It would be nice if the extension didn't just look at the characters after the last period in the URL to determine whether or not it's looking at a markdown file - note, this is just my assumption from observed behaviour; I haven't actually looked at the code (yet). For example, I have a script that can take a URL parameter which is the name of a file, and that script itself converts markdown files to html. Unfortunately, markdown-preview then kicks in and incorrectly renders the resulting html.
I realise that not everyone serves markdown files with an appropriate mime-type, but it would at least be nice to make this behaviour configurable.