Enabling user-supplied matplotlib colormaps

Adding a configurable ColormapRegistry which allows the user to supply
their own matplotlib colormaps for the Image Viewer in the config.py
file. The already existing colormaps are loaded in by default.
Moving matplotlib import inside function and docstring
Simplified syntax for adding colormaps
Merge pull request #198 from jzuhone/master

Enabling user-supplied matplotlib colormaps