make runner.live_plot work with a wider range of backends

[jbweston]

the docstring says:

plotter : function
    A function that takes the learner as a argument and returns a
    holoviews object. By default ``learner.plot()`` will be called.

But not everyone wants to use holoviews as a plotting backend!

[jbweston]

e.g. This is a (pretty sick) example that uses K3D