Add smart_passive collector

Commit adds smart_passive collector which collects metrics from pre-scraped
data. The reason for doing it this way:

1] take away logic, which is often complicated, how to actually get SMART data
2] SMART data reusability. Often, more than just one monitoring tool is using
SMART data, eg. Diamond and Nagios. This way, you query your disks only once
instead of N times.
Can you document more about how to generate the specific output format here?