The encoding module should be an optionalDependencies #675

The encoding module is conditionally required, i.e. should be an optionalDependencies in the package.json.

Doing so allows libraries that parse require() statements (using JavaScript AST) such as precinct to know whether a require() statement that cannot be resolved indicates an error or not.

This causes problems on our library zip-it-and-ship-it: netlify/zip-it-and-ship-it#16, netlify/zip-it-and-ship-it#30