Add option to allow members of whitelisted organizations to whitelist users/trigger PR tests

This adds a config option to allow members of whitelisted organizations to effectively behave like they were entered into the admin list, which is more practical for organizations with many members.

We are using this at the Mono project since a few days and it seems to work fine :)
A much cleaner way would be to ignore all comments from the bot user.
Indeed that was the case in the past (see https://github.com/janinko/ghprb/issues/25), but the ```repo.isMe()``` check was removed in 1b0767ce2bfb709bab8e3cde6c11d54205487aea, so someone could now rely on that behavior.
I'm not 100% happy with the check I introduced, as I imagine it could fail if text/character encoding or some other issues prevent the string from being equal.

@valdisrigdon Do you think it is better to change that again?