[feature request] Add dependency change only mode

Running into this with the [ember-decorators](https://travis-ci.org/ember-decorators/ember-decorators/) library. Currently, the library is set up as a monorepo that consists of many addon packages, managed by `lerna`. This setup works well, but is very slow for testing. Even after consolidating all of the tests into the "main" `ember-decorators` package, it still takes twice as long as it would for a standard addon.

This is due to a double install occuring - first, `lerna` runs its install on every package via `lerna bootstrap`, and then links them up. Second, we run `ember try:one` in the main addon, which installs everything again.

I think ideally we would add a mode where you could only apply the diff to `package.json` in a given addon. Then, in our case, we would run the diff command first, then run `lerna bootstrap`, and have everything be installed and linked correctly in one go.

I'd be willing to open a PR for adding this if it seems like an acceptable feature.