Add cri stats

[Random-Liu]

Fixes #138.
Based on #208.

Note that we'll sample for 1 second for cpu percentage by default. The sample duration could be changed with --sample.

Help:

$ crictl stats --help
NAME:
   crictl stats - List container(s) resource usage statistics

USAGE:
   crictl stats [command options] [arguments...]

OPTIONS:
   --all, -a                  Show all containers (default shows just running)
   --id value                 Filter by container id
   --sandbox value            Filter by sandbox id
   --label value              Filter by key=value label
   --output value, -o value   Output format, One of: json|yaml|table
   --seconds value, -s value  Sample duration for CPU usage in seconds (default: 1)

Output for running containers:

$ crictl stats
CONTAINER           CPU %               MEM                 DISK                INODES
303cdce60ee9a       0.07                13.43MB             28.67kB             8
43b34bf65eb15       0.00                2.712MB             57.35kB             16
e0f6114591a9e       0.23                11.69MB             32.77kB             9
f8f112011f784       0.00                7.299MB             28.68kB             9

Output for all containers:

$ crictl stats -a
CONTAINER           CPU %               MEM                 DISK                INODES
26b3f9397a5f6       0.00                0B                  16.38kB             6
303cdce60ee9a       0.08                13.43MB             28.67kB             8
43b34bf65eb15       0.00                2.712MB             57.35kB             16
e0f6114591a9e       0.17                11.69MB             32.77kB             9
f8f112011f784       0.03                7.299MB             28.68kB             9

Filter for specific sandbox:

$ crictl stats --sandbox f4c59c78c6dbb
CONTAINER           CPU %               MEM                 DISK                INODES
303cdce60ee9a       0.02                13.43MB             28.67kB             8
e0f6114591a9e       0.15                11.69MB             32.77kB             9
f8f112011f784       0.00                7.299MB             28.68kB             9

[feiskyer]

/lgtm