Use a more reasonable name.
Now 'call' method can accept *one* argument at interactive mode.
(Hopefully) more graceful exit.
More logical command name.
Rearrange functions in a logical order.
Add a test file to test UI module.
Now commands in the built-in shell can take arbitrary number of
parameters.
Now `call` should be able to executed recursively.
More comprehensive docstring.
Simply logic in `parse_input` method.
More flexible way to invoke the method of a submodule.
Now `help` can get `msg` of a submodule.