Support funny exit codes

In order to support arbitrary exit code to monitoring status mapping,
provide one option for each severity level to setup a mapping.

The default Nagios style mappings are still kept. If the Nagios style exit codes are not remapped, they keep their severity.

closes https://github.com/Jimdo/periodicnoise/issues/27 and https://github.com/Jimdo/periodicnoise/issues/28

@mlafeldt PTAL
@Luzifer please take a look at the tests at https://github.com/Jimdo/periodicnoise/blob/5a7c83fa471937077ed5da500b08b1fd3821c5ef/cmd/pn/flags_test.go#L67-L98 whether they describe the puppet behavior.