Plot Blocking Option

[stlgolfer]

In matplotlib, there is a block argument that dictates "whether to wait for all figures to be closed before returning." However, the ternary/ternary_axes_subplot.py wrapper does not appear to allow for this. I am currently working on research that heavily uses the python-ternary library, and having the blocking option would greatly help me out. I'm happy to fork and PR if that makes the process easier!