Support discord relay for multiple simultaneous channels/guilds

[cupanoodle]

The config could looks something like this and allow sending messages to separate discord guild/channels.

message-types:
  join: primary
  leave: secondary
  chat: [ primary, secondary ]

currently the settings are just read in a a single string:

Essentials/EssentialsDiscord/src/main/java/net/essentialsx/discord/DiscordSettings.java

Line 86 in 680d323

public String getMessageChannel(String key) {