Added tab complete.

[KyGuy2002]

Show Subcommands:
/show list list is tabcompleted
/show start start is tabcompleted, as well as all shows that arent currently running.
/show stop stop is tabcompleted, as well as all shows that are currently running.

Showgen Subcommands:
/showgen setcorner setcorner is tabcompleted as well as the players current coords. Also, I added support to the command so you can use a standard coord format, rather than x,y,z you can now do x y z. Old is still supported for backwards compatibility though.
/showgen setinitialscene setinitialscene is tabcompleted .
/showgen generate generate is tabcompleted, as well as fakeblock and bottom/top, but only if you start typing so both formats work.

Other:
I also fixed an out-of-bounds exception when not proving a show name, and any unknown/incomplete commands return a default help message.

My only concern is when generating the tab completions for show start, it has to actually check for files, so it could cause lag. I tried to spam it and there were no issues, plus it's a staff-only command so I don't think it will be a problem.

This should be ready for review, but I will update you if I find any issues. Thanks!

[KyGuy2002]

Alright @CubitsDev i removed a dupe filename check I noticed, but for your other requested changes I have left comments responding to them. Thanks