[Feature Request] Add a setting for automatic "Now playing" information when a song starts playing

**Is your feature request related to a problem? Please describe.**
When a song starts playing the bot doesn't automatically show which one. That can be a little bit annoying when doing something else while the bot is playing in the background.

**Describe the solution you'd like**
When the bot starts a new song, it automatically posts the song title similar to the "Now playing" command

**Describe the basic flow/steps of using this feature**
How this feature would be used:
1. Turn on the feature via a command like "!setautonowplaying"
2. Bot starts song
3. Bot posts "Now playing" information (Title, song length)

Start by tracing how commands are registered and where playback begins a new song, then compare that flow with the existing "Now playing" command. Confirm how a per-server setting would be stored and toggled. Done means the setting can be enabled with a command and the bot posts the song title and length whenever a new song starts; check linked pull request #1010 first.

Written by the indexing model from the issue text.