New API Debug Mode. 

If enabled, any exceptions handled by Dingo Router will add an additional debug key filled with exception information.

This PR aims to solve the issue brought up by #69. There are somethings I am not sure about, such as the Config facade use, or the adding of HTTP Request information.

Up to discussing further to refine this. We could make this super basic, or even go as far as to write in custom formatting for the trace output. But don't want to overkill this.
I'm assuming your alternative will be a setter method on the class?