Add diagnose or debug or auth command to debug user config

[4-20ma]

I use 2 different users on github.com. I'd like to use one of the following commands to debug which user/auth is being used and where the info is coming from (I'm not picky about the name of the command--just provide me with the ability to use multiple usernames/auth and tell me where it's getting the info):

$ hub diagnose
<provide output similar to test-kitchen's kitchen diagnose command--essentially it provides the deep-merged yaml output>
...

$ hub debug
...

$ hub auth
...

[mislav]

This is a good idea, and already planned: #225