Plot the cash at the broker

### Expected Behavior

We should have a flag for the `bt.plot()` function which would show the cash amount at our broker.

E.g.: `bt.plot(plot_broker_cash=True)`

Also, given this feature it would be nice to access the "cash at the broker" in each iteration.

Thank you for the great work!

Start at the bt.plot() entry point and trace how broker state is represented during each iteration. Define the behavior for a plot_broker_cash=True option and for accessing broker cash per iteration; done means both requested capabilities work consistently in a backtest.

Written by the indexing model from the issue text.