Feature: Decrease CRTITCAL ERRORS to WARNING

Hi Elan

I don't know if I didn't understand the --warnonly option correctly.

There are two diffrent explanations for this Option
In your readme:
```
-W  --warnonly          Don't send CRITICAL status
```
and in the code.
```
Treat CRITICAL errors as WARNING
```

What I experienced with the -W option, is that CRITICAL is changed to UNKNOWN.
According to your Readme this is may be correct.

From the explanation of the --help option, I thought a CRITIAL ERROR will be decreased to WARNING.

Without any option:
```
# perl check_raid.pl
CRITICAL: mdstat:[md1(927.52 GiB raid1):UU, md0(203.81 MiB raid1):F:sda1:_U]
```
And with --warnonly option
```
# perl check_raid.pl -W
UNKNOWN: mdstat:[md1(927.52 GiB raid1):UU, md0(203.81 MiB raid1):F:sda1:_U]
```

My pullrequest adds the --nocritical option.
The ERROR level for a CRITICAL ERROR will be decreased to WARNING.

Without any option:
```
# perl check_raid.pl
CRITICAL: mdstat:[md1(927.52 GiB raid1):UU, md0(203.81 MiB raid1):F:sda1:_U]
```
And with  option
```
# perl check_raid.pl -C
WARNING: mdstat:[md1(927.52 GiB raid1):UU, md0(203.81 MiB raid1):F:sda1:_U]
```