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).
Merge pull request #959 from benjamn/commoner-multiple-file-support

Upgrade Commoner to v0.8.14 for multiple file support.