include file in body endpoint return

[avelino]

Specifying large returns in the same moclojer configuration file (.yaml or any format) is "boring" and makes the specification file difficult to read.

Why not support the file inclusion tag for the body?

- endpoint:
    method: GET
    path: /external-body-text
    response:
      status: 200
      headers:
        Content-Type: application/json
      external-body:
        provider: text
        path: test/moclojer/resources/text-plan.json

using the specification external-body created for xlsx support #152

[avelino]

#161