Provide access to the pillow Image object used by export_png() and export_svg()

[monkut]

I'm using bokeh 0.13.0 with python 3.6 to auto-generate charts in AWS lambda periodically to send to Slack.

In order to achieve this I want to use bokeh.io.export_png() to export the generated chart to a PNG.
However, the AWS lambda environment is mostly read-only, and I would like to have direct access to the underlying image object used to save the '(png|svg)' so that I could write directly to a BytesIO() object instead of writing/reading to/from disk.

Currently, I've done a quick hack to allow this:

def export_png(obj, filename=None, height=None, width=None, webdriver=None, as_image=False):
    image = get_screenshot_as_png(obj, height=height, width=width, driver=webdriver)

    if filename is None:
        filename = default_filename("png")
    if as_image:
        return image
    else:
        image.save(filename)
        return abspath(filename)

[bryevdv]

That's not a hack, get_screenshot_as_png is the correct function to use if you want access to the image as an object.