MATLAB's "zplane" like plot of a Filter

[pineapplehunter]

Hi.
I am taking a course in university aboud DSP and I wanted a plot of a filter that looks like the "zplane" plot in MATLAB.
I could not find a way to plot the filters in this repo so I made my own code for it. It's in this gist.
https://gist.github.com/pineapplehunter/bf75b8d67a1d6859dfabd8dc7d987c31

The output plot looks like this.

I don't know if this repo is the right place to put this code, but if it is I'd like to make a PR. If a PR sounds good where should this code be in the source tree?

[martinholters]

Ref. #203 and #226. FWIW, I'd also consider a dependency on RecipesBase acceptable.

Another issue is that we should probably support a better API for accessing the poles and zeros than via the internal fields of ZeroPoleGain.

[tapyu]

Is this support already implemented? This pzplane matlab-like worked like a charm. It definitely should be added to the DSP.jl