Add invert option to allow checking if a value is less than a threshold

We needed the ability to check whether certain stats were below a threshold rather than equal to or greater than.  I added an invert option that will do just that.  Take this traditional check:

```
./check_redis connected_clients -H <host> -w 6 -c 10
```

In the normal operation this would alert warning if there were >= 6 clients and critical if >= 10 clients.  If you simply add a -i option (and reverse the w and c thresholds so they make sense):

```
./check_redis connected_clients -H <host> -w 10 -c 6 -i 
```

we can now alert warning if there are less than 10 clients and critical if there are less than 6 clients (Not checking equal to on inversion since it's checked on the normal operation)

I don't know if you'll find this useful for inclusion in this project but I wanted to contribute upstream in case.  Thanks for your consideration!