[Feature Request] Run `scripts` for all configured workspaces concurrently

Since Npn has added workspaces (and Yarn has them too), it would be nice to have a "concurrently" version of those.

For instance, with npm run --workspaces test, the test package.json#scripts will be run for all workspaces sequentially.

A "concurrently" version of the above might be: concurrently --workspaces npm:test.