Play along nicely with Markdown's Meta extension

Before, this plugin didn't play nicely if Meta was also enabled (which is by default e.g. in [Pelican](http://blog.getpelican.com/)); `self.markdown.Meta` was overwritten by Meta. Also the resulting `Meta` object was non-conforming to the established Meta API (namely all keys mapping to `list`s). So that's both fixed as well as some other minor things.