cli: support specifying a transport for neighbor command
cli: support specifying a transport for neighbor command

Added '-t' (--transport) option, which can take "ipv4" or "ipv6":

fujita@ubuntu:~/git/gobgp/gobgp$ gobgp neighbor
Peer           AS  Up/Down State       |#Advertised Received Accepted
10.0.255.1  65001 01:59:53 Establ      |          1        2        2
10.0.255.2  65002 01:59:53 Establ      |          2        1        1
2001:db8::7 65007    never Active      |          0        0        0

fujita@ubuntu:~/git/gobgp/gobgp$ gobgp neighbor -t ipv4
Peer          AS  Up/Down State       |#Advertised Received Accepted
10.0.255.1 65001 02:02:20 Establ      |          1        2        2
10.0.255.2 65002 02:02:20 Establ      |          2        1        1

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

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