add option to operation commands to set header value(s)

[gdereese]

Changing the method used to execute the operation to using Swagger.buildRequest() then feeding into Swagger.http() will work great for paving the way for this feature.

However, not really sure what string format would be good for representing the header names and values in an option, as these didn't work out:

• --headers name=value (headers with - in the name were truncated)
• --headers "name"="value" "foo"="bar" (only the first header name got include, the rest was truncated)