Upgrade Commoner to v0.8.14 for multiple file support

See [Commoner's README.md](https://github.com/benjamn/commoner#generating-multiple-files-from-one-source-module) for further explanation of the new functionality.

Among other potential benefits, this upgrade allows us to generate source maps as standalone files rather than appending them inline to every compiled module file under `build/modules/` (see #833).

The `commonerConfig.version` bump isn't strictly necessary, but it seems like a good idea whenever we change the way Commoner works internally.

cc @andrewdavey @zpao