Consider adding a 'prepare' script to build when npm installing from git repo

[onebytegone]

With lambda-express in early development, features and fixes are getting added and need to be tested faster than new versions are being cut. As such, it would be handy to be able to
install lambda-express directly from the git repo. However, due to the build process and needing the dist files, this isn't currently possible using npm i <repo>. Fortunately, a prepare script was added in npm 5. This script is ran when installing from a git repo and can be used to build the needed dist files.

More info: https://blog.jim-nielsen.com/2018/installing-and-building-an-npm-package-from-github/#installing-and-building-packages-with-npm-from-github

[onebytegone]

Closed with #40