[Feature Request] "Synced lyrics" from subtitles of the video

**Is your feature request related to a problem? Please describe.**
Not really a first world problem, but some people may fancy synced lyrics of a video when playing it back with the Music bot.

**Describe the solution you'd like**
Several YouTube videos have subtitles available & I got the thought of displaying the subtitles (of the current timestamp) in the `nowplaying` command. This could be a pretty neat feature to implement (especially into the `nowplaying` command since it updates every 5 seconds. Perhaps we could make it update every time new subtitles are being shown?)

**Describe the basic flow/steps of using this feature**
1. Play a YouTube video with subtitles
2. Use the `nowplaying` command
3. Watch as the subtitles update as the music plays back

**Additional context**
None.

**Have you confirmed that this idea is not already described here?**
https://github.com/jagrosh/MusicBot/projects/1
Yep.

Start with the nowplaying command and inspect how YouTube video playback and timestamps are handled. Define the subtitle timing behavior, then verify that a video with subtitles displays the current subtitle and updates as playback advances.

Written by the indexing model from the issue text.