export: Improve design and test coverage of export

This commit:

- Moves responsibility for I/O back into `honcho.command` rather than
  `honcho.export`.
- Changes exporters so that `#export()` takes a list of
  `honcho.environ.ProcessParams` objects and optional template context.
  This means that concurrency, process naming, and port assignment is
  now much more likely to be consistent between `honcho start` and
  `honcho export`.
- Improves test coverage of the `honcho.export` package.
- Removes a couple more fragile export integration tests.