Improve design and test coverage of export package

This PR:

- 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.

It does, but it's template-specific: https://github.com/nickstenning/honcho/pull/119/files#diff-28f114616a4e57b7cd22e1e78902cff8R13