Support multiple file outputs for `cog predict`

[bfirsh]

It currently writes a file to disk if there's a single output, but outputs plain JSON if there are multiple files.

We should instead write those files to disk using a sensible naming scheme.

Migrated from a todo in #378.

Proposal for how to handle this here: #328

edited by @zeke

[andreasjansson]

The old multiple-output proposal had some ideas for how to handle this: #328