[Feature Request] Forceskip/Skip Volume Fadeout

**Is your feature request related to a problem? Please describe.**
The abrupt change when using skip and forceskip can be jarring.

**Describe the solution you'd like**
The ability to set in the config that on skipping the current playing song it will instead fade out. Preferably the setting would be "Fade duration by seconds", with setting it to 0 working how it is now.

**Describe the basic flow/steps of using this feature**
How this feature would be used:
1. Set config option "fadeout" to desired number of seconds
2. Run skip or forceskip

**Additional context**
If there are any other commands, like the suggested playnow ( https://github.com/jagrosh/MusicBot/issues/155 ), should also be affected by fadeout if implemented. That way it would be kept consistent across commands.

**Have you confirmed that this idea is not already described here?**
Didn't see anything that looked remotely in suggested or planned. Even did a search within issues with no filters.

Start by locating the configuration handling and the command entry points for skip and forceskip; also check the suggested playnow issue for related behavior. Trace how the current song is stopped and identify the existing tests or playback paths that cover these commands. Done means a configurable fade duration, with zero preserving current behavior and the affected commands behaving consistently.

Written by the indexing model from the issue text.