execute code from config file

[manishtomar]

Since config is basically python file, there should be facility to specify functions that will get executed to generate parts of request or headers. For example

  "headers": {
        "content-type": "application/json",
        "accept": "application/json",
        "X-Auth-Token": get_auth_token
    },

get_auth_token will be a function that will generate auth token every time.