Publish specific files and folders to NPM

Users of `scratch-render` do not need the `playground` directory
installed as a npm package. If they need those directories they can
clone scratch-render and do a local build. This saves downloading 3.5MB
(`playground`) when `npm install`ing `scratch-render`.

.npmignore

@@ -0,0 +1,16 @@
+# Development files
+.eslintrc.js
+/.editorconfig
+/.eslintignore
+/.gitattributes
+/.github
+/.jsdoc.json
+/.travis.yml
+/test
+/webpack.config.js
+
+# Build created files
+/playground
+
+# Exclude already built packages from testing with npm pack
+/scratch-render-*.{tar,tgz}