enabled markdown arguments in extension

I don't know Django so I don't know whether this functionality is desirable, but I thought I'd send a pull request anyway and see what you think.

My fork allows the extension to take markdown arguments in a dictionary assigned to a template environment attribute. Since jinja extensions are initialized upon environment definition, the only way environment data attributes could be passed into the `markdowner` was by moving the `markdown()` instantiation out of `extensions.__init__` and into `extensions.parse()`. An example of how this works is provided in a separate file.

I'm quite new to programming and this is my first pull request, so any feedback would be appreciated.