Allow passing errorhandler to the npm request module used internally.

[gautam1168]

The npm 'request' module supports passing in a callback function that is called after the request is completed with the result or the error. There is no way to pass this callback/handler currently. This is required because making a call to an non-existent url using express-request-proxy throws an error that cannot be caught by the caller program. I have made a PR to add this: #42