Degrees formatting for angles

For example:

```python
data = {
'x': [0, 1],
'y': [0, 1],
'a': [0.79, 3.93],
'r': [.71, .35],
}
ggplot(data, aes('x', 'y')) + \
geom_spoke(aes(angle='a', radius='r'), \
tooltips=layer_tooltips().line("angle|^angle")) + \
coord_fixed()
```

In this case, it would be nice to be able to apply formatting that replaces the angle in radians with an angle in degrees.