CDAP-506: Allow CLI completers to access context

I have changed version of common.cli from 0.1.0 to 0.2.0-SNAPSHOT, because this changes require 0.2.0-SNAPSHOT version of common.cli. @jwang47 Is this right choice?
I don't like having to write this block of code every time I want to access the context. Can you think of some way to accomplish this without having to write a new `*PrefixCompleter` every time I want to access the context for a command?