"todo list" should be same as "todo ls"

This is not a really pull-request. Note that my problem won't fix with this pull-request.
I'm using this `todo` but sometimes I mistake to list my todo.

![](http://go-gyazo.appspot.com/316367ec2f70ef31.png)

I'm thinking this is because this command-line is not uniformed as human-interactives. So it should change sub-command. For example:

### Require sub command. Not add with "todo foo".
```
$ todo add "foo bar baz"
$ todo list
```