Support for in-buffer variables

Like we talked about in #16 :-) Here from the updated README:

You declare a variable like this:

    :myvar = the value

After the var is declared, you can use it in the URL, the header values
and the body.

    # Some generic vars

    :my-auth = 319854857345898457457

    # Update a user's name

    :user-id = 7
    :the-name = Neo

    PUT http://localhost:4000/users/:user-id/
    Authorization: :my-auth

    { "name": ":the-name" }

Warning: If you include var declarations as part of the request, in
the body or headers, it will be sent along.

Instead, place them above your calls or in separate sections. Like in
the example above.