Decouple formatador from STDOUT

We added the ability to pass an IO to `Formatador.new`. We wanted to use Formatador for generating a table in a plain-text email, but currently can't, because it only writes to STDOUT.

This would seem to address part of Issue #14.

The specs are currently failing, and we're not sure why, but suspect it has to do with the Threading in the `capture_output` helper.