Global dependency installs for virtual environment

還沒有人認領這個 Issue。

I know it's not really the done thing, but I use a virtual environment ([nodeenv](https://github.com/ekalinin/nodeenv)) for my projects, and would very much like to have the option not to have a `/node_modules` folder, but instead have required packages installed with the global flag. This would actually install them to my virtual environment.

Is there a way of doing this? If not, could the option be added?

該 issue 提到了 nodeenv、/node_modules 和全域套件安裝，但沒有指出任何 Ionic CLI 檔案或測試。先找出 CLI 的相依性安裝入口和現有的選項涵蓋範圍；完成條件是提供一個有文件說明的選項，將必要的套件安裝到虛擬環境中，並為該行為加入測試。

由索引模型根據 Issue 內容生成。