Support static screenshot mode including widgets

[droumis]

Currently hv.output(example_holomap, fig='png') will not return a 'png' of the plot + widget as a '.png' - what I would call a 'screenshot' mode. I don't believe there is currently a way to do this.

This functionality would come in handy when building a notebook that displays incremental steps in building an application, but you do not want interactive versions of the intermediate steps of the application, and you don't want to have to save a png to file.

Code

import numpy as np
import holoviews as hv
hv.extension('bokeh')

frequencies = [0.5, 0.75, 1.0, 1.25]

def sine_curve(phase, freq):
    xvals = [0.1* i for i in range(100)]
    return hv.Curve((xvals, [np.sin(phase+freq*x) for x in xvals])).opts(tools=['hover'])

curve_dict = {f:sine_curve(0,f) for f in frequencies}

hmap = hv.HoloMap(curve_dict, kdims='frequency')
hv.output(hmap, fig='png') # same interactive result as just running `hmap`