[feature request] support pouchd --version command line

[allencloud]

Issue Description
Currently I failed to execute pouchd --version with error message:

root@ubuntu:~/go/src/github.com/alibaba/pouch# ./pouchd --version
Error: unknown flag: --version
Usage:
   [flags]

Flags:
  -c, --containerd string          where does containerd listened on (default "/var/run/containerd.sock")
      --containerd-config string   Specify the path of Containerd binary (default "/etc/containerd/config.toml")
      --containerd-path string     Specify the path of Containerd binary (default "/usr/local/bin/containerd")
  -D, --debug                      switch debug level
  -h, --help                       help for this command
      --home-dir string            The pouchd's home directory (default "/etc/pouchd")
  -l, --listen stringArray         which address to listen on (default [unix:///var/run/pouchd.sock])

root@ubuntu:~/go/src/github.com/alibaba/pouch# ./pouchd version
ERRO[2017-11-03 14:54:28.002306216] failed to new containerd's client: failed to connect containerd: failed to dial "/var/run/containerd.sock": dial unix /var/run/containerd.sock: connect: connection refused
INFO[0000] starting containerd                           module=containerd revision=20c621178a5f45938e71afad24df281002aac329 version=v1.0.0-beta.1
INFO[0000] setting subreaper...                          module=containerd
INFO[0000] loading plugin "io.containerd.content.v1.content"...  module=containerd type=io.containerd.content.v1
INFO[0000] loading plugin "io.containerd.metadata.v1.bolt"...  module=containerd type=io.containerd.metadata.v1
ERRO[2017-11-03 14:54:28.010396816] failed to wait process: /usr/local/bin/containerd, signal: killed
WARN[2017-11-03 14:54:28.010530606] process: /usr/local/bin/containerd exited
Error: failed to new daemon
Usage:
   [flags]

Flags:
  -c, --containerd string          where does containerd listened on (default "/var/run/containerd.sock")
      --containerd-config string   Specify the path of Containerd binary (default "/etc/containerd/config.toml")
      --containerd-path string     Specify the path of Containerd binary (default "/usr/local/bin/containerd")
  -D, --debug                      switch debug level
  -h, --help                       help for this command
      --home-dir string            The pouchd's home directory (default "/etc/pouchd")
  -l, --listen stringArray         which address to listen on (default [unix:///var/run/pouchd.sock])

We still missing this part.

Describe what happened:
failed to execute pouchd --version

Describe what you expected to happen:
pouchd --version works to return version info of pouchd

How to reproduce it (as minimally and precisely as possible):

1. install pouch
2. execute pouchd version

Anything else we need to know?:
NONE

Environment:

• pouch version (use pouch version): NONE
• OS (e.g. from /etc/os-release): Ubuntu 16.04.3 LTS
• Kernel (e.g. uname -a): Linux ubuntu 4.4.0-97-generic [docs] Missed Usage and System Design Documentation in README #120-Ubuntu SMP Tue Sep 19 17:28:18 UTC 2017 x86_64 x86_64 x86_64 GNU/Linux
• Install tools: NONE
• Others: NONE

[allencloud]

#dibs