Define packags needed in package.json, add a command to copy the
distribution files to the js/vendor/ folder in gulpfile.js and then
it's just a matter of doing:

1. Updating version numbers in `package.json`
2. Running `npm run update-libs`

But just the distribution files

spin.js==2.3.2 and jszip==2.6.0 need to be pinned to these versions
otherwise we need to upgrade shp2geojson (which seems unmaintained)

Just use a bash script to copy the distribution files from the node
modules folder to the vendor one, which was essetially what gulp was
doing. Store all files at the library root level (no dist/ or src/
folders)