Additional request header parameters for httpjson plugin

[asosso]

This will provide the ability to specify header parameters for httpjson plugin.

Example:

[plugins.httpjson.services.headers]
  X-Auth-Token = "my-xauth-token"
  apiVersion = "v1"

[sparrc]

Thank you @asosso! I'm going to wait until 0.3.0 is done to get this merged, see #437

[sparrc]

@asosso There are now significant changes in master, can you please rebase?

[sparrc]

@asosso Could you try making this change on a fresh branch?

[asosso]

@sparrc yes, of course.

[sparrc]

thank you @asosso 👍