Added Flow

[notatestuser]

Requested in #36.

Flow is a static type checker for JavaScript. More on Flow:

Type Inference
Using data flow analysis, Flow infers types and tracks data as it moves through your code. You don't need to fully annotate your code before Flow can start to find bugs.

JavaScript, Your Way
Flow is designed to understand idiomatic JavaScript. It understands common JavaScript patterns and many of the weird things we JavaScript developers love to do.

Realtime Feedback
Flow gives you fast feedback while you code by incrementally rechecking your code as you make changes.

Easy Integration
Flow integrates well with many tools, making it easy to insert into your existing workflow and toolchain.

Implementation notes:

No changes to the existing build scripts. Already caught a couple of issues in utils. It's entirely opt-in through adding a comment // @flow on the first line of a file and you can start with //@flow weak and fix up the errors reported by npm run lint until the flag can be removed to reveal the remaining errors.

There's a extension for VSCode that works pretty well and displays coverage as you code:

This is only covering src/utils.js for now. More to come!