Adds command tab completion control

Command Tab Control
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Too many plugins do not implement permission correctly, and many servers end up having malicious users snooping around the server's commands by tab-completing command names, deducing which plugins are being used and many might even find a command that wasn't protected correctly to abuse, and to top it off - it doesn't even log it.

This lets server admins control if to allow tab completion for command names using a flag "command-tab-complete" in bukkit.yml.

__Bukkit PR:__ https://github.com/Bukkit/Bukkit/pull/804