New package.json field, "peerDependencies"

Just like "dependencies" hash, but:

1. Each entry must be installed in the family (at the same level or higher) as the package. (If the package is the top target, and not global, then install as normal deps, maybe, just for testing/development purposes?)
2. If not found, then fail, unless --force flag is set.
3. Never ever include in the package when publishing.

This enables express middleware and other plugin-type things to specify that they depend on a certain version of express being in-play, but not bundled underneath them.

Open questions:

Should peerDeps be installed by default if missing? Leaning towards no, since this brings up all kinds of cleanup issues on uninstallation.