What problem does this feature solve?

Some plugins might want to depend on other plugins. Currently, there is no way to do this.

What does the proposed API look like?

If the package.json dependencies of vue-cli-plugin-a contains vue-cli-plugin-b, then:

1. vue add a will run vue add b and then vue add a
2. vue invoke a will run vue invoke b and then vue invoke a (Optionally configured by a)

We can do this multiple deep levels.

Issues

1. If a depends on b and c, what's the order of b and c to be run?
2. What about circular dependencies?