Handle PGRES_COPY_BOTH in psql for logical replication?

Hello Hackers,

At the moment one can open up a replication connection using psql and use
all of the commands available on this special type of connection, except
for START_REPLICATION:

$ psql -d "dbname=test replication=database"
psql (9.4.1)
Type "help" for help.

test=# IDENTIFY_SYSTEM;
systemid | timeline | xlogpos | dbname
---------------------+----------+-----------+---------
6146106447402521313 | 1 | 0/76422B0 | bw_test
(1 row)

test=# START_REPLICATION SLOT test LOGICAL 0/0;
unexpected PQresultStatus: 8
test=#

This is due to PGRES_COPY_BOTH not being handled in psql. With
introduction of logical replication I think it makes sense to support this
mode, for the sake of (semi-)interactive testing from psql.

For example, one could try replication from different wal positions,
different output plugins and/or plugin options, w/o re-establishing the
connection, interactively by editing the command directly in psql prompt,
as opposed to using pg_recvlogical and editing the command line switches.

I can make a patch if there's general interest for this feature.

Cheers.
--
Alex