Add a config option for the default music bot volume

[Jokler]

Is your feature request related to a problem? Please describe.
When a music bot it has a default volume preset, this volume is often too loud.

Describe the solution you'd like
A config option would allow the default volume to be changed per server.

Describe alternatives you've considered
Lower the default volume for everyone but since this is subjective I am against it.

Relevant line

pokebot/src/bot/music.rs

Line 94 in 67c1189

player.change_volume(VolumeChange::Absolute(0.5)).unwrap();