Add makeKeyProp API to allow users to generate their own React key props

[anthony-dandrea]

The problem:

The way we currently generate React key props is either by using the node's id or generating a random string key={child.id || randomString()}. This isn't ideal because:

1. Users aren't guaranteed to have an id key/value. They could be using any number of arbitrary key names such as key, sha, etc for their unique key name.
2. Generating a randomString() for a key prop isn't any better than using index as the key prop(arguably worse), which is an anti-pattern. This is because the keys aren't consistent for every render, causing React to re-render all the children it is mapping over.
3. Having non-consistent keys every render will cause performance hit on larger trees.
4. Having non-consistent keys can potentially impact animations. (See https://github.com/storybookjs/react-treebeard/issues/224)

The solution:

I added a makeKeyProp escape hatch for this case. This will allow users to write a function that returns a key based on the data of the respective node. This change is backwards compatible as there is a fallback as a defaultProp to use the current method.

Notes:

• Also helps users address this issue: https://github.com/storybookjs/react-treebeard/issues/224
• Fixed unrelated failing test in __tests__/Container.test.js