command/lock: Pass stdin to child process when -pass-stdin passed.

[ryotarai]

I added -pass-stdin option to pass stdin to a hander process.

$ echo foo | bin/consul lock bar cat

$ echo foo | bin/consul lock -pass-stdin bar cat
foo

Use case

I'd like to execute an arbitrary command via consul lock on events fired.

$ consul watch -type event -name foo \
  consul lock -pass-stdin bar cat
[{"ID":"4deac22a-7f00-5320-8de1-2ee5932f4599","Name":"foo","Payload":null,"NodeFilter":"","ServiceFilter":"","TagFilter":"","Version":1,"LTime":2}]

[slackpad]

Hi @ryotarai - code looks good. Can you please also update https://github.com/hashicorp/consul/blob/master/website/source/docs/commands/lock.html.markdown as part of this change?

[ryotarai]

Hi @slackpad
Thank you for reviewing. I updated the doc: ryotarai@b2755d0