Support code-folding of debug output

[Chris Warburton]

It's a good idea to dump out ephemeral information as we go, in case
it's useful for diagnosing an error which occurs later. For example, we
might print out the content of a file, then try parsing it, etc.

Yet this data is useless when things work properly, which (if we're
doing it right) should be most of the time.

Since we use Emacs to run commands, perhaps we could delimit such
messages in a way that Emacs will automatically code-fold them. That way
they're there if we need them, but we can skip over them when we don't
care.