Added "rhc env-var [list|add|remove]" commands to handle user settable environment variables
Added "rhc env-var show" command, syntax adjustments
Now handling empty :arg_type == :list as nil arg, env var now supporting a list of variables, improved descriptions
Moved app to an option instead of argument on all rhc env commands except list
rhc show-env
Unset multiple env vars
rhc set-env now handles files as input