Allow the TCP CONNECTIONS column to be shown on all stat queries in the CLI.
This column will now be called TCP_CONN for brevity.
Read/Write bytes will still only be shown on -o wide or -o json
Added support for json output in `linkerd stat` through a new (-o|--output)=json option.
Fixes#1417
Signed-off-by: Alejandro Pedraza <alejandro.pedraza@gmail.com>