cli: support showing the state of single neighbor
cli: support showing the state of single neighbor

$ gobgpcli show neighbor <neighbor_ip>
e.g)
$ gobgpcli show neighbor 10.0.0.2

Signed-off-by: FUJITA Tomonori <93dac1fe9c4b2a3957982200319981492ad4976e@lab.ntt.co.jp>

h-naoto/gobgp,a16/gobgp,ramrunner/gobgp,tamihiro/gobgp,a16/gobgp,a16/gobgp,h-naoto/gobgp,osrg/gobgp,fujita/gobgp,a16/gobgp,ramrunner/gobgp,Dipsingh/gobgp,a16/gobgp,osrg/gobgp,osrg/gobgp,fujita/gobgp,fujita/gobgp,tamihiro/gobgp,tamihiro/gobgp,YujiOshima/gobgp,h-naoto/gobgp,Dipsingh/gobgp,ishidawataru/gobgp,ishidawataru/gobgp,YujiOshima/gobgp,postfix/gobgp,j-kato/gobgp,YujiOshima/gobgp,j-kato/gobgp,fujita/gobgp,tamihiro/gobgp,ramrunner/gobgp,osrg/gobgp,postfix/gobgp,postfix/gobgp,ishidawataru/gobgp,j-kato/gobgp,Dipsingh/gobgp,a16/gobgp,a16/gobgp