Consolidate and change strategy with the export formatters.

Now there are two formatters, since module-variable has been removed:

- commonjs: This is essentially unchanged. It still does rewriting of
  references to imports, but does not rewrite references to exports or
  locally-declared variables & functions.
- bundle: This is almost the same as what was the 'export-variable'
  formatter. The difference is that all locally-declared variables &
  functions at the top level will be renamed to avoid collision with
  top-level declarations in other modules. We do this because we've
  dropped the IFFEs that previously surrounded each module's contents,
  which will hopefully allow for even better tree-shaking and
  minification.