Previously eslint ignored .jsx files :|

eslint-config-scratch: React + ES6 Edition

node relevant at the root, but not in src. Broser relevant in src, but not in root.

null !== undefined

Pin React devDependencies, add peerDependencies

When the package is installed as a dependency, it should warn the installer that React is a peerDependency.  When installed this way, the devDependencies won't be installed, so only one version of React will be present. However when it's installed as a standalone project, React should be installed automatically as a devDependency.

I think this gives us the best of both worlds — simple development setup as well as supporting usage as an installed package.

Initial rough sprite/costume/backdrop libraries

# Conflicts:
#	package.json
#	src/containers/gui.js

Makes local deployment easier

So that you can adjust them without redefining the entire GUI.
Also allow adjusting props passed to GUIComponent.

So that it's easier to use these standalone.

`npm update` does what we want and is less hacky than removing the packages entirely first.  Specify latest as the version so we always get the latest version even though I erroneously installed scratch-blocks@0.1.0.

And use instanceOf for the vm prop type