Add success and output return values to chat commands

This is useful for capturing the output of commands, for example to reply to a IRC channel with it or save the output of the last command.
The success return value can be used for stringing commands together conditional on success like shell's `&&` operator (My textarea commandblock could make use of this).
Example usage: https://gist.github.com/ShadowNinja/48b263aead046086a75d