load check settings via ENV

So I had this idea that it would be nice to access the check's custom key/values (and maybe other stuff) from within the check plugin. Right now only handlers get access to the data, but it could be used in the checks themselves for configuration or other behavior changes in addition to command line options.

Not sure about naming it "settings". Might be confusing with the existing "config" used for command line options. Any other ideas for a name?

In addition to this PR, it requires a 1 line change for sensu-client to set the ENV before running the check. The only downside is an additional Oj.dump for each check execution. If that's a problem there could be a flag to turn it off.