check-stats.rb - added option to ignore UNKNOWN

If you use check-stats.rb to wildcard-match a ton of metrics at once (like, CPU usage from a whole fleet of instances), sometimes there will be one or two instances that are missing data for various reasons. That will throw the whole check into status UNKNOWN, whether you care about those stragglers or you don't.

I've added option -u or --unknown-ignore to let the plugin ignore those few metrics with an UNKNOWN status, and return proper status for everyone else.

If you don't use -u, you get the old behavior.
Merge pull request #885 from FlorinAndrei/patch-6

check-stats.rb - added option to ignore UNKNOWN