Add per-server prefixes mod

[TheMonDon]

Please describe the changes this PR makes and why it should be merged:
Adds a per-server prefix mod that lets you have a different prefix per server

Status

• Code changes have been tested against the Discord API and the discord.js wrapper, or there are no code changes
• Documentation has been added/modified, or there is nothing to change (docs/mods.json)

Semantic versioning classification:

• This PR changes DBM's interface (methods or parameters added to default methods)
• [] This PR only includes non-code changes, like changes to documentation, README, etc.