Add support for @p, @a and @r selectors.

[dualspiral]

Selectors require the nucleus.<command-name>.selector permission - which will be different for each command.

The following selectors, some of which are Nucleus specific, are available:

• @p - nearest player to command executor, not including command executor
• @p[world,x,y,z] - nearest player to co-ordinates, including command executor
• @r - random player, not including executor
• @r[world] - random player on specified world, not including executor
• @a - all players
• @a[world] - all players on world.

So far, the following commands have selectors. Only /smite has access to the @a family of selectors:

smite
msg
burn
hat
tppos
fly
spawnmob
tpa
tpahere

Support for /tp will be coming up soon for @p only.

Other commands can be added on request.

Fixes #317