[feature request] streaming part of the webpage

[imbible]

Puppeteer can capture a screenshot of a part of the webpage by using CSS selectors. It will be awesome if puppeteer-stream can provide similar feature for streaming a part of the webpage by using CSS selectors. I understand #132 asked this as a question and found a workaround which worked for the specific webpage that the author tested, but I doubt the workaround will work ubiquitously on most website, so I believe this can be feature to provide in puppeteer-stream. Let's take the sample code for example. A possible scenario is that the user only wants to stream the Youtube video but not the whole page.