Added sync/async whitelist hooks options

[webocs]

# Whitelist Hooks

For a use case we have, we needed to whitelist by user role and not by user IP. Thus, I added a new option called whitelistHook and whitelistHookSync. The objective of such options is to provide the user a mean to whitelist using whatever criteria she needs. It uses the same logic that onDenial.

Changes

• Added whiteListHook option
• Added whiteListHookSync option
• Added behavior for both options, this required splitting the _handle method in two
• Added tests for both options
• Updated readme

Final remarks

I figured I'd PR it since it can be useful to others, be free to accept or reject

Cheers!