Add ability to call other redis commands

This commit adds the ability to get metrics from other redis commands. In particular, I wanted to be able to check the length of a key list (LLEN key). In keeping backward compatibility I called this option "operation".  Usage might look like:

```
./check_redis <key> -H <host> -w 20 -c 25 -K -o llen
```
This would call `@redis.llen(<key>)` and would compare the results to w/c values of 20/25 respectively. 

I am open to suggestions/changes if you think something would be better suited in a different way.  Thanks for taking a look.