Convert matplotlib stairs plots to plotly step lines

[robertoffmoura]

mpl_to_plotly currently drops plt.stairs plots entirely. Matplotlib's stairs creates a StepPatch artist, which the exporter treats as a generic path, and the renderer only handles bar-chart paths, so the stairs plot is skipped with the warning "I found a path object that I don't think is part of a bar chart. Ignoring." The converted figure has zero traces.

Fix: draw_path now recognizes StepPatch artists and draws them as a line trace:

• the StepPatch's path vertices already describe the horizontal/vertical step outline, so they're used directly (consecutive duplicates removed)
• line styling (color, width, dash) comes from the patch's edge properties

Snippet to reproduce:

import matplotlib
matplotlib.use("Agg")
import matplotlib.pyplot as plt
import numpy as np
import plotly.tools as tls

x = np.linspace(0, 10, 20)

fig, ax = plt.subplots()
ax.stairs(np.sin(x))
fig.savefig("stairs_mpl.png")

p = tls.mpl_to_plotly(fig)
p.write_image("stairs_plotly.png") 

matplotlib	plotly before	plotly after