Support for using this as a nagios plugin

[dnmvisser]

As it says on the tin.
It would be handy if this could be used as a nagios plugin. This requires a specific output format, so a feature flag would be suitable (--nagios or something)

[fschulze]

Aren't there already plugins for that? If not, then a PR is welcome, as I have no idea or interest about nagios myself.

[dnmvisser]

There are those plugins already, but they either are too simple, or way too heavy. Your script seems to have a reasonable set of features and still is light.