WiP: Video capture

[pablochacin]

What?

Implements a video recording API for the browser using the screencast API to capture screens and ffmpeg for converting them into a video (similar approach used by puppeteer's screen recorder)

The functionality to start recording a video is added to the page interface. As it is not possible to have more than one active recording at a time, it was decided not to return a screenrecorder object as in the puppeter's API , but leave the functionality to control the active recording (basically, stop the recording) in the page object itself. Addopting pupperter's model would be relatively simple.

Why?

• Capture what the end-user experience is like
• Help debug flaky tests that fail due to unforeseen changes in the page under test

Todo

• Fix: output directed from ffmpef to the file persister is truncated
• Ensure video is recorded if the page is closed
• Review API (method names, etc)
• Add tests
• [ ] Review style
• Document
• Add Examples

Checklist

• I have performed a self-review of my code
• I have added tests for my changes
• I have commented on my code, particularly in hard-to-understand areas

Related PR(s)/Issue(s)

Closes #103