Add command-line option to generate treemaps instead of scatter plots

So I've been talking up this tool for a long time, and everyone says "Kerri, it sounds cool, but that graph is kinda hard to understand!"  

![scatter_plot](https://f.cloud.github.com/assets/257689/1061321/408a2852-11ce-11e3-849c-75ee07199987.jpg)

Well, to each their own! However, once I started piping the output into a treemap graph, I came to really like the clarity and "idiot light on the dashboard" quality that the treemap graph offered. 

![treemap](https://f.cloud.github.com/assets/257689/1061324/800741e0-11ce-11e3-8ca8-41380d0b26bf.jpg)

Although I did rummage the files around a bit, original functionality is preserved, simply adding a ```--treemap``` option to the command line to generate treemaps instead of scatter plots.