Log players joining/leaving the server

[mrghosti3]

Add functionality on top of Discord webhook (#2) to send notification on who is playing minecraft.

Tasks:

• Implemented basic messages for player logging
• Implemented message formatting
• Check whether player rejoined after getting connection error and send a message accordingly if not rejoined
• Don't send message of player joining the game if they rejoined after network error

[mrghosti3]

Color for the embed message #ab274f

[mrghosti3]

Progress as of 0a34bfe:

Currently I have implemented stateless message sending which is not ideal, as it doesn't cover a case where it doesn't send that a player left when they were kicked out or disconnected due to network issue. Ideally this could be solved by queuing up a task to check whether the player has returned to game and if not then sending OFFLINE message.

[mrghosti3]

For now I'm pushing the updates to master tagged as v0.2.1.

Short version of commit:
Made some changes to internal workings. Using already existing logger from extended JavaPlugin class. No more message MsgType enum for storing static premade messages, now it's just constants for when server going online or offline. The rest will be made accordingly when they are needed (ex. player joins). Removed unused code.

[mrghosti3]

Added another improvement to this, which now send those messages asynchronously, which takes less resources away from main thread.