[C+B] Implement command tab completion in the console. Adds BUKKIT-4168

##### The Issue:
The default implementation of Command.tabComplete() didn't allow anything but Players to tab complete.
Also the SimpleCommandMap.tabComplete() always prepended "/" to commands, which is not needed in the console.

##### Justification for this PR:
This PR is required for the feature to work

##### PR Breakdown:
- removes the Player check from Command.tabComplete()
- prepends the "/" only for Players
 
##### Testing Results and Materials:
Testing this is straight forward: Enter something (or nothing) into the console and press tab, it works pretty much exactly like ingame
 
B-868 - https://github.com/Bukkit/Bukkit/pull/868 - Command(Map) changes to allow console tab completion
CB-1151 - https://github.com/Bukkit/CraftBukkit/pull/1151 - Completer implementation
 
##### JIRA Ticket: 
BUKKIT-4168- https://bukkit.atlassian.net/browse/BUKKIT-4168