[Suggestion] Add npm-cache to speed up 2nd+ install

[rafibomb]

It will cache everything locally so it will be super fast on the 2nd install.

[jamesstoneco]

Here is the difference from my limited testing via the cli which should be similar to a manual npm install.

foundation new --framework sites --template zurb --directory test 167.51s user 53.12s system 113% cpu 3:14.51 total

./test-npm-cache.sh 9.92s user 5.53s system 98% cpu 15.638 total

So, 3:15 vs 15s, huge speed difference, especially on the zurb template.