Provide a way to add custom stepper components

### Description

It looks like Spectacle supports two steppers only: Appear and Stepper.
There are cases when none of the above is a good fit.
For instance: integrating https://github.com/pomber/code-surfer, or drawing a plot where step elements result in adding more data or adding lines to the plot.

### Proposal

What if there was a way to declare a number of steps explicitly or with a hook?

For instance:

```
// This configures the slide to have 5 steps, and the current step could be requested as usual context

```

Alternative option: provide a hook that would enable a component to declare the number of underlying steps.
For instance, in the cases like `code-surfer`, the number of steps is defined by the markup, and it would be error-prone (and inconvenient) to specify the number of steps twice (in code-surfer markup and in ``.

### Links / References

https://github.com/jxnblk/mdx-deck/blob/0cca3a082414ca4b186ae1e7ecbda8ab00ab11bf/docs/api.md#usesteps