Add temp-ban feature

[CapacitorSet]

Extend the ban plugin to allow for temporary bans.

[avipatel91]

What would the command look like?
`/ban username days:hours:minutes

e.g./ban foobar 07:42:00

[CapacitorSet]

It would be nice if one could use a more "natural" notation like 20d for "twenty days".

By the way, I think such a plugin would benefit from a scheduling API. Pinging @crisbal for discussion.

[crisbal]

It is definitely a feature we could work on in the future.

Right now there are other priorities (porting the old dev plugins to the new es6 branch for example or adding feature to the Util modules).

[CapacitorSet]

Depends on #93.

@Ichicoro is working on a scheduling API that should allow us (well, her) to implement both the Events plugin and this one. Consider this issue taken.