Allow getitem from a cycler

It would be useful to add `__getitem__` functionality to `cycler`.

The use case is a "crowded" for loop where you want to get the cycler style without using a loop variable. Something like:

```
data_cycler = cycler('alpha', (0.2, 0.4, 0.6)) + cycler('color', ('r', 'g', 'b'))
fit_cycler = cycler('color', ('k', 'm', 'orange'))

for i, (data, fit) in enumerate(zip(data_list, fit_list)):
...
plt.plot('x', 'y', data=data, data_cycler[i])
plt.plot('x', 'y', data=data, fit_cycler[i])
...
```

In this case the index `i` is computed anyway, so using it for the cyclers would be handy, instead of adding the cycler inside `zip()`.

Ideally `mycycler[i]` should return the item with index `i % len(mycycler)`.

I haven't looked at the code yet. Would this be possible? Would a PR be welcome?

Start by reading the cycler implementation and its existing tests, focusing on how cycles are iterated and how their length is exposed. Add indexing with the requested modulo behavior, and update tests to show that mycycler[i] returns the item at i % len(mycycler).

Written by the indexing model from the issue text.