Implement fancy indexing for GraphicFeatures

We need to implement fancy indexing for indexable graphic features so that these things are possible:

```python
line = plot.add_line(array, colors=(0., 0., 0., 0.))

line.colors[array > 5] = "r"
```

Can probably be implemented in `cleanup_slice` by using the mask and getting the indices, `np.where(array > 5)` etc.
