Send overly long output to a paste service, the post URL in channel

[david-christiansen]

IRC discussion:

> Melvar: feature request for idris-ircslave: sending gigantic output to a paste service instead of concatenating
> then putting url in channel
<Melvar> christiansen: Measure giant how?
> i dunno
> perhaps more than 3 lines?
> more than 5?
> something like that
<Melvar> christiansen: Maybe you should open an issue, so I don’t forget … also since I truncate ordinary output if too long, perhaps I should do that then too. As in, if you have more than 300 chars, I’ll show that many, then an url to a paste of the whole thing.