Nagios Plugin Support

This adds polish the Nagios support from #26:

* specifying nagios support via the `--nagios` flag
* handling file errors w/the config file (not found, permission issues, etc)
* not running commands w/remote arguments, which we can't access. 

## Code notes

* Most of the logic needed in `Scout::Server` is abstracted into the `ThirdPartyPlugins` module. I didn't want this to dirty-up the existing code. 
* There are separate `NagiosPlugin` and `MuninPlugin` classes. The initialization of these is easier as they don't have memory or options. 
* The Munin logic still needs some more error handling and testing.