Adds the ability to specify additional whitelist users from the command line

chkcrontab has been an awesome addition to our CI process, but we have a few custom users in our crontabs that trigger warnings.  Rather than ignore these types of warnings, I wanted to be able specify specific users which should also be whitelisted (in addition to those specified in code).  

The users are specified as so...
$ ./chkcrontab my_crontab --whitelist=user_1,user_2