Dynamic commands

• New commands to return NBT/Entities/Blocks (e.g. /getentity /getblock, /data get already exist)
• Bracket-like syntax <> to put commands in (e.g. "/summon pig ~ ~ ~ {Passengers:[{id:</data get etc.>}]}")

As in the example above this would achieve chances like spawn-time control on NBT data, like in case of Passengers, and new commands to detect ID of blocks and entities. This could also represent an alternative method to traditional one-commands.