fix ability to run a single plugin file

@btm pointed out that the ohai application would break when using the `-f` command. I've now updated the ohai application to run a single file.

I've marked a TODO item on [line 102](https://github.com/opscode/ohai/blob/c7043950e65bc927e52eedbe83b983cc8a9a25d3/lib/ohai/application.rb#L102-L103). The help description on [line 33](https://github.com/opscode/ohai/blob/c7043950e65bc927e52eedbe83b983cc8a9a25d3/lib/ohai/application.rb#L33) is the most non-invasive fix I could come up with for now. Not sure if we want to do anything about it. @danielsdeleo