Pass custom built MPL colormaps

Feature request: allow passing custom made Matplotlib colormaps

```py
import matplotlib.pyplot as plt
cmap = plt.cm.get_cmap("viridis", 5)
```

then pass that colormap to the `Viewer`

(the goal isn't necessarily catecorical colormaps... this is just a simple example. A user might want to make a custom normalized map that are far more complex)