2.x api for setting ffmpegpath

This is a followup to PR #276, which will be now closed.

What has been done in this PR:

* The following methods have been turned to static and made public (used to be private)
   - this._getFfmpegPath() --> Ffmpeg.getFfmpegPath()
   - this._getFfprobePath() --> Ffmpeg.getFfprobePath()
   - this._forgetPaths() --> Ffmpeg.forgetPaths()
* The rest of the codebase and tests have been updated to match that change.
* Two new static methods where introduced:
   - Ffmpeg.setFfmpegPath([string])
   - Ffmpeg.setFfprobePath([string])
* Tests where authored for the new functionality
* README was updated