Implement command tab completion in the console. Adds BUKKIT-4168

Even though JLine provides an API for tab completion, there was still now tab
completion for console commands.
This commit implements tab completion in the console by providing the
ConsoleReader with a Completer implementation.
To accomplish this 2 changes to Bukkit were required:
- the player check in Command.tabComplete() had to be removed
- the SimpleCommandMap prepends the / only for players