Ignore knife subcommands from other chef installs

Fixes https://github.com/opscode/chef-dk/issues/227

I will add regression tests soon. We also need to manually test this when installed as a gem, to ensure the logic to allow subcommands from within the same gem version is correct. And please inspect the regexes carefully to see if you can find a flaw in them.

/cc @opscode/client-engineers 
maybe add a 'foo-chef-1.2.3/lib/chef/knife/whatever.rb` case and make sure it shows up to ensure the regexp anchor doesn't regress.