Added support for horizontal graphs

I was looking around for a good looking method for producing a git commits graph a webpage, and this project is by a wide margin my favorite. It did lack a built-in way to produce horizontal graphs though, which I prefer for using in blog posts, so I patched in a way to do that easily. I thought you might be interested in this feature as well, so here is a pull request with those changes, including new examples and explanations of how it works. I hope you can find it useful.