added headers option - work in progress

I'm trying to add headers support to coap-cli.  I was thinking that it could be similar to payload i.e.

coap get -h "skynet_auth_uuid=123&skynet_auth_token=abc" coap://coap.octoblu.com/status

The -h headers option has been added as well as the parsing logic but I am having trouble actually sending the headers with the coap call.  Here is the line(s) in question.

https://github.com/chrismatthieu/coap-cli/blob/master/index.js#L75

Please let me know if you spot my issue(s).

Thanks,
Chris