Allow commonjs modules to use module.exports directly

When using CommonJsModuleTransformer, the module can either use the import statement, or the "old" require() function. This allows the same thing for exports. If a module doesn't use any export statements, it can still export values the pre-ES6 way by assigning to exports.foo or module.exports. Traceur users can migrate to ES6 export statements gradually instead of all at once.

Fixes #598.

Review URL: https://github.com/google/traceur-compiler/pull/629

Closes #629, closes #598.