fix `command -h` problems

[dusmart]

You can reproduce the problems by using help -h twice.

The problems when using command -h are

1. duplicate help message after using -h
2. cannot read default value of the option properly after using -h

This PR fix them by

1. prevent adding options that have the same name as existing ones
2. prevent modifying option.usage from Object to string

[dusmart]

http://terminal-in-react.surge.sh/

color white color -h color -h color -c red