ENH: Supply a list of systems for time domain plotting

[ilayn]

This is slightly more convenient for plotting all systems at once. Still cuts to the shortest response if no time array is given to avoid excessive sampling. I don't remember what matlab or octave was doing. I also don't know what to do with legends and leave it to user via kwargs. So

G = random_state_model(6, 3, 2)
F = random_state_model(6, 2, 3)
H = Transfer(100, [1, 100, 100])
J = discretize(G, 0.1)
step_response_plot([F, G, H, J])

gives

impulse response is also similar.

Note: tox stuff is still broken, add root-locus and that makes v1.0.2 (finally!).