[CB-4077] Separate the actions of removing a plugin from a platform and ...

...removing the plugin entirely

This patch should be coordinated with the matching pull request for cordova-cli.

This patch keeps the original uninstall() interface, and adds two new methods: uninstall.uninstallPlatform(), which removes a plugin from a single platform, and uninstallPlugin(), which removes the plugin from the project's `plugins/` directory.