Issue 18 add help flag

Add a --help flag to the command line.                                                          
                                                                                                
This prevents the error `hi: unrecognized option '--help'`.                                     
                                                                                                
This is for issue https://github.com/fujimura/hi/issues/18                                  
but is based off of the code for https://github.com/fujimura/hi/pull/20                         
so https://github.com/fujimura/hi/pull/20 should be merged                                      
before this is merged.