Added duration argument type.

[sriRacha21]

An attempt at resolving issue #182 .

Takes a string like this: 1w 2d 3h 4m 5s (this would represent 1 week, 2 days, 3 hours, 4 minutes, 5 seconds).

All fields are optional and can be supplied in any order.

parse() returns the amount of time in milliseconds and validate() ensures there is at least one time code present in the string and that the represented time is over 5 seconds (I've noticed some wonkiness with any actions that happen within 5 seconds so I've set this limit. Feel free to remove this).

I've also added the type and a command to the test folder to make it easier and quicker to give it a shot. The command takes a duration as an argument, sends a message, and edits the message after said duration.

[ghost]

If I am not wrong, there are no changes in the registry, which allows to register, enable or disable this Type.

Can this also be added, so that the type can be registered also? (Needs Updating of Typings also in that case)

[ArnaudLier]

Duplicate of #256