Allow existing charge to be captured

This PR is not ready for integration, but I thought I would submit the code for the sake of clarifying what I'm trying to accomplish.

Stripe's [documentation](https://stripe.com/docs/api/ruby#charge_capture) provides the following example for capturing an existing charge:

```ruby
ch = Stripe::Charge.retrieve("ch_2KqJkK1jF4deHa")
ch.capture
```

This method will return the original charge hash, but with the `'captured'` key set to `true`.

I've been trying to implement this behavior in stripe-ruby-mock, but I can't seem to find the right hook to trigger any behavior that I implement. I think I'm missing something very basic. So far I've tried imitating the code for Customer#update_subscription, and other instance methods, but I'll spare you further details.

Where should I start to intercept the Charge#capture method?