Add 4 more bridge commands

We now can interact with a bridge by adding / removing channels and
adding / removing holds.

   bridge-channel-add
   bridge-channel-remove
   bridge-hold-add
   bridge-hold-remove

Change-Id: I207d3d5a0f898dbe494f6549b40e093dd9c0f062
Signed-off-by: Paul Belanger <paul.belanger@polybeacon.com>