Allow using server name as a variable in chat, join, and leave messages

[unilock]

might be a good idea to make changes to default config as well
closes #2

(had to update maven-shade-plugin to get it to build)

[fooooooooooooooo]

wicked

[unilock]

I opened a similiar pull request on velocity-chat, and the author suggested that it would make more sense to listen to ServerConnectedEvent rather than PlayerChooseInitialServerEvent for detecting when a player joins a backend server, as the latter is meant to be used when wanting to change the server a client is connecting to.

This makes sense to me, and it would allow merging the onConnect() and onServerConnect() listeners as well (as ServerConnectedEvent fires whenever a player joins a backend server via the proxy, regardless of whether they were already connected to the proxy). Though I'm not 100% sure if this would be desired?