Update time-remaining.js: add Playback Rate

[muescha]

What:

• Added playbackRate to the time remaining calculation.

Questions:

1. Should this be implemented in the current plugin, or should it be a separate plugin independent of the time-remaining plugin? A separate plugin would be code duplication, but might more clear in the options dialog.
2. Should it be added as an additional option to time-remaing, similar to "time_remaining_position," with a setting like show_playbackRate: yes/no? However, this would limit the customization of its position in the template string.
3. Should playbackRate be included in all format templates where speed is relevant?