Add `label2rgb`

This PR adds a function to overlay colorized labels on top of an image. You can see the results of this function in the modified `plot_join_segmentations.py` example.

Not included in the example: You can also use a matplotlib colormap to choose colors. For example:

```python
colors = plt.cm.Set1(np.linspace(0, 1, 12))
colorized = label2rgb(label, image=image, colors=colors)
```