check-smart.rb plugin

`check-smart.rb` basically does the same thing as `check-disk-health.sh` does, but there'd be small differences:

* `--smart-incapable-disk` option can be used to specify the behavior when there is a SMART incapable disk.
    * `check-disk-health.sh` doesn't inspect the capabilities and exits with critical if there's a such disk.
* Verbose output. `check-smart.rb` outputs the command output in addition to the status.
* Written in Ruby. I think it's easier to extend it later.