It's been soon™ for years. The dev has been fairly quiet about it, but popped now and then on his blog or twitter with new deets.
It was initially supposed to be just a small addition to the game, but has ballooned, and will now nearly double the game's size when it finally drops. Each time I've checked though, there's been some news.
And when I did just now to get links to the new music and gameplay gifs that I found last time, I just now found out you can try out some new content in the game using a secret beta branch on steam. Its right there in the news feed if you go look at the game in your library.