Command Format: add a new command `list`
Command Format: add a new command `list`

`list task|event|due` can be used to list all kinds of tasks by passing
in different parameters.

The tasks that are listed will be ensured not expired.

CS2103AUG2016-T11-C4/main,nightism/addressbook-level4,snehasp13/main,CS2103AUG2016-T11-C4/main,snehasp13/main,nightism/addressbook-level4