publish --canary should have some way of limiting publish to changed packages only

Expected Behavior

In much the same way that lerna publish behaves (by only publishing packages with changes, plus packages with dependencies on those), lerna publish --canary should have some way to restrict publishing to only those packages with changes.

Current Behavior

Currently, publish --canary forces inclusion of all packages (here, in UpdatedPackagesCollector.js)

Possible Solution

Use the --since flag to filter out unchanged packages.

Context

In a project with a large number of packages, being able to create --canary builds on only packages that have actually changed since the last release would save significant time. In my project, this would reduce the build time by at least 80%.