Added support for all Nagios statuses in output

Previously the nagios output only supported issuing:

* WARNING, or
* CRITICAL

for check results.

This commit adds support a config option, nagios_level, which
defaults to CRITICAL and can be overriden by the "nagios_level"
field in an event.
Updated with feedback from Jordan
Merge pull request #298 from jamtur01/nagioslevel

Added support for all Nagios statuses in output