Add the "coords" kwarg to plot_autocorr

[jaredsagendorf]

Tell us about it

Unlike most of the plotting functions, for some reason plot_autocorr doesn't support the coords kwarg. Unless this was intentionally left out, seems like it would be more convenient to include this option.

Also, there is no reference to the grey band that is automatically drawn anywhere I could find. What does this interval represent? The plotting function provides no ability to toggle this, which it should.

[SuramyaP]

Can I work on this?