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.

Example config.py:

```python
from yt.mods import *
from yt.visualization.color_maps import yt_colormaps
from glue.config import colormaps

# This is a standard matplotlib colormap
colormaps.add("Jet",cm.jet)

# These are yt-based matplotlib colormaps
colormaps.add("Algae",yt_colormaps["algae"])
colormaps.add("idl29",yt_colormaps["idl29"])
```