Add alpha parameter to plot_bands

[lwasser]

it would be ideal to be able to do:

fig, ax = plt.subplots(figsize = (10,6))
ax.imshow(lidar_dem_hill, cmap='Greys')
fin_plot = ax.imshow(lidar_dem_im, cmap='viridis_r', alpha=.5)
fig.colorbar(fin_plot, fraction=.024, pad=.02)
ax.set_axis_off()
ax.set(title="Lidar Digital Elevation Model (DEM)\n overlayed on top of a hillshade")
plt.show()

fig, ax = plt.subplots(figsize = (10,6))
ep.plot_bands(lidar_dem_hill, cmap='Greys', ax=ax)
ep.plot_bands(lidar_dem_im, cmap='viridis_r', alpha=.5, ax=ax, title="Lidar Digital Elevation Model (DEM)\n overlayed on top of a hillshade")
plt.show()

so essentially we just need to add alpha as a parameter.

[lwasser]

completed via #412