Added method for printing a graph of Unicode characters.

Hello, I wrote a small method for creating Unicode graphs. You can see an example of how it could be used here - https://github.com/facetoe/i3pystatus_modules/blob/master/cpu_usage_graph.py. I'm also trying to make them change colour based on the values but that is in the early stages. My current method makes the graph cycle through brown on it's way from green to red which is, well, pretty crappy (no pun intended!).

Btw I've just changed from i3status to i3pystatus and I really like it so far, it's a great project. Thanks!