* allow user to set dockerhub credentials
* add withJwtAuth function to AuthHelper
* use withJwtAuth in DockerHub badges
* add unit tests for JWT auth
* use auth when calling docker cloud
* refactor and assert fetch helpers call withJwtAuth
* store token for a max duration (defaults to 1 hour)
* tangent: update test example
* allow serviceData to override cacheSeconds with a longer value
* prevent [endpoint] json cacheSeconds property exceeding service default
* allow ShieldsRuntimeError to specify a cacheSeconds property
By default error responses use the cacheLength of
the service class throwing the error.
This allows error to tell the handling layer the maxAge
that should be set on the error badge response.
* add customExceptions param
This
1. allows us to specify custom properties to pass to the exception
constructor if we throw any of the standard got errors
e.g: `ETIMEDOUT`, `ECONNRESET`, etc
2. uses a custom `cacheSeconds` property (if set on the exception)
to set the response maxAge
* customExceptions --> systemErrors
* errorMessages --> httpErrors
