Support for `--force` flag when installing plugins or themes

Use case: I'm testing some upgrade scripts and need to reset my local environment to an out of date state. As such, I'd like a `--force` flag to make wp-cli install a specific version of a plugin or theme, regardless of what's installed or active.

Another use case: Plugin X has known issues with its latest version, so you want to roll back to its previous version.

Current workflow is two parts:

```
wp plugin delete co-authors-plus
wp plugin install co-authors-plus --version=3.0.4 --activate
```