The injection of App.vue might be unnecessary

After `vue add vue-cli-plugin-iview` and all the installation steps, the plugin would inject and modify my origin App.vue and showing the demo of Button.
Showing an example is quite good but I have to reset the modification of App.vue if I have already modified it.
Maybe offering an option when installing is better?

Start by tracing the `vue add vue-cli-plugin-iview` installation flow and where it modifies the project's `App.vue`. Clarify the intended installation option and whether the default should preserve an existing file; done means the chosen behavior avoids unexpectedly replacing user changes.

Written by the indexing model from the issue text.