[Feature] Allow using "{current}" variable in other contexts

[unilock]

Currently you can only use the {previous} and {current} variables in the server_switch_message. However, it would be useful to be able to use these variables in other messages, especially the latter.

Some examples:
({username} = "player", {current} = "vanilla", {previous} = "modded")

~

This works fine:

server_switch_message="**{username} moved from {previous} to {current}**"

Result:

player moved from modded to vanilla

~

This does not work:

join_message="**{username} joined {current}**"

Result:

player joined {current}

Desired result:

player joined vanilla

~

Another case where this would be useful:

message="[{username}@{current}] {message}"

Desired result:

[player@vanilla] hello, world