Optional Catch Binding (Stage 4)

Champion: @michaelficarra
Spec repo: https://github.com/michaelficarra/optional-catch-binding-proposal/
Spec text: https://michaelficarra.github.io/optional-catch-binding-proposal/
Stage: -1 :)
> Moved to Stage 3 at the July meeting https://github.com/tc39/agendas/blob/master/2017/07.md

> Stage 4 at May 2018 meeting

This hasn't been presented to the committee as of this writing, but will be at the [July meeting](https://github.com/tc39/agendas/blob/master/2017/07.md). If it receives broad support it is likely to advance quickly, in my personal estimation.

It allows the binding to be omitted from a `catch` clause:

```js
try {
throw 0;
} catch {
doSomethingWhichDoesntCareAboutTheValueThrown();
}
```

or

```js
try {
throw 0;
} catch {
doSomethingWhichDoesntCareAboutTheValueThrown();
} finally {
doSomeCleanup();
}
```

This will need a (very small) change in Babylon, including a change to the AST (marking the catch binding as optional), and then a very small change to Babel.

This would make a great first contribution, I think!

**UPDATE:**
- [x] Babylon PR - https://github.com/babel/babylon/pull/634
- [x] Babel PR - https://github.com/babel/babel/pull/5956

The issue points to the Babylon PR #634 and Babel PR #5956, both marked complete. Start by reviewing those linked changes and the optional-catch-binding proposal to verify the implementation and its AST handling; done means the proposal is supported by both projects and the relevant tests pass.

Written by the indexing model from the issue text.