Add bash script to download necessary files for Colab

[davidwitten]

I have been running these notebooks in Colab instead of Jupyter notebook and I have had to include a script like this for past assignments:

%%bash

if [[ ! -d "./data" ]]
then
    echo "Missing extra files (this probably means you're running on Google Colab). Downloading..."
    git clone https://github.com/cs124/pa3-ir.git
    cp -r ./pa3-ir/{data,porter_stemmer.py} .
fi

This script is present in pa0 and pa4.