The name of the key in the webhook json response should be configurable

[smarter]

According to https://colineberhardt.github.io/cla-bot/#configuration-options

With each invocation, the checkContributor querystring parameter is used to supply the committer username. The webhook should return a JSON response that indicates whether the committer has signed a CLA:

{ "isContributor": "true" }

Instead of requiring the json response to contain a key named isContributor, it would be nice if it was possible for the user to specifiy the key since they might be interfacing with a service that uses a different convention.