Tidy up `cog predict` output

[bfirsh]

It's much more messy than it should be. This is a follow-on from #155.

[bfirsh]

There should be a bit more space around output, as this commit demonstrates: replicate/cog-examples@308c031

That commit really is a Cog bug that is being compensated for with the model output.

[anish-sahoo]

Closing as this was addressed by PR #2793, which added visual separators and icon-prefixed framework messages to distinguish model output from Cog's own output.