truncation for too many items

[Almenon]

Creating gigantic lists means renderjson has to render thousands of even millions of items, which is too much for the html renderer to handle without lag.

I raised a feature request in caldwell/renderjson#22 for this, though if I have the time I might try doing it myself.

[Almenon]

I tried developing this for a tiny bit. Got it sort-of working but the tabs are wonky.

see Almenon/renderjson@cc0b12a