screenshot.py changes: Html2Image logging and option to discard the html screenshot

[jtenkate]

Hi, when I recently used the library for the first time, I encountered two things in the screenshot lib that gave me issues:

1. by default class Html2Image generates terminal logging. As I'm using this log to monitor test progress, I want to disable this (unnecessary) logging
2. when using the keyword Take Screenshot with the png option, the html screenshot is also generated. I don't need both, so I want to have an argument to discard the html if a png is created.

Let me know what you think. I have a changed file screenshot.py ready if you agree to this change.

Cheers, Jeroen