[standalone] Support authentication in standalone mode

[deathcap]

For other servers (standalone mode)

If your server does not support Bukkit or Sponge plugins, then WSMC can be ran standalone from the command-line:

java -jar target/wsmc*.jar 0.0.0.0 24444 localhost 25565

User authentication is not supported in this mode.

Authentication is only currently supported in the Bukkit and Sponge plugins, because they have API hooks to allow getting when a new player joined (to generate their new key), receiving commands (/web), sending messages. Standalone/vanilla support has no such interaction with the target server. But it may be able to support manual setup, through a file (username -> login key).

Could be useful to investigate after #42 Persist the authentication key table to disk