Add keyboard shortcuts that will increase/decrease the playback rate for video elements.
Updated version of the old video-element-rate-controller, with some added enhancements, such as having configurable settings for the adjustment rate as well as being optimized to have a lower memory footprint.
By default, "[" will slowdown by 0.1x, "]" will speedup by 0.1x, and "\" will reset the speed to 1x. This speed value can now be adjusted in the configuration menu for the script. Let me know if you have problems now that localStorage is being used.
In the future, I may make the key that changes the speed to be configurable.
Tested working with Mozilla Firefox 62.0b12 (32-bit) and Mozilla Firefox 61.0.2 (64-bit).
