add help output to the command line client

Add some helpful output to the command-line client, in case you don't pass a parameter or you pass an invalid parameter. My intention was to do this without adding any additional gem requirements, so it's fairly basic. I'm also not 100% certain that putting the 'help_exit' method in the bin/unimidi file is idiomatic ruby, but I couldn't think of a better place to put it.