Automatic package management and clickable actions

[forrestbao]

This is what DeepNote does.

1. If I am trying to import a module that is not installed, it will give me a clickable link (blue font over yellow background). Once clicked, it will create and execute a new cell to install the missing module.
2. Once the module is installed, it will give me a clickable link to add the module dependency into requirements.txt.

We shall support this.