Add plot range zooming

Hello purveyors of triangle.

I added a simple plot-range zooming keyword to my triangle fork.  I find this useful to combine with `extents=[0.99]*N`, when some parameters have hard edges in their distributions.  It's nice to see the edges more clearly with `zoom=1.1`.

Compare:
`triangle.corner(data, extents=[0.99]*3)`
![nozoom](https://cloud.githubusercontent.com/assets/3650485/2726025/66284590-c5b4-11e3-9ccf-735780c3167a.png)
`triangle.corner(data, extents=[0.99]*3, zoom=1.1)`
![zoom](https://cloud.githubusercontent.com/assets/3650485/2726026/66285e4a-c5b4-11e3-8a22-41e7d1adac03.png)

If I'm missing a simpler way to accomplish the same thing, let me know!

-Josh