Make this package smaller

[justinbmeyer]

du -sk * | sort -n | tail -r
536	demo
184	ndjsonStream.gif
16	dist
8	docs
4	test.js
4	test.html
4	test-sauce-labs.js
4	readme.md
4	package.json
4	license.md
4	index.html
4	can-ndjson-stream.js
4	can-ndjson-stream-test.js
4	build.js
4	CONTRIBUTING.md

We should not publish the demo folder and ndjsonStream.gif. Add these to .npmignore.

Useful: https://docs.npmjs.com/misc/developers#testing-whether-your-npmignore-or-files-config-works