lein-midje-doc will not run without option "plain"

Just wanted to check your plugin out and since nothing was generated, even when I tried it with the lein-midje-doc project itself, I went on a (really small) expedition into your code and fixed this small bug.

Also, I'm really looking forward to some of the features on the wishlist, e.g. single-page generation, markdown rendering and themes. Great job so far!