Pipeline Operator (Stage 1)

Champion: @littledan
Spec Repo: https://github.com/tc39/proposal-pipeline-operator
First presented at Sept 2017 meeting: https://github.com/tc39/agendas/blob/master/2017/09.md
Slides: https://docs.google.com/presentation/d/1qiWFzi5dkjuUVGcFXwypuQbEbZk-BV7unX0bYurcQsA/edit#slide=id.p
Related proposal: Partial Application https://github.com/rbuckton/proposal-partial-application by @rbuckton
Related proposal: Function Bind Operator https://github.com/tc39/proposal-bind-operator

## Basic Example

### Input

```js
let result = "hello"
|> doubleSay
|> capitalize
|> exclaim;
result //=> "Hello, hello!"
```

### Output
```js
let result = exclaim(capitalize(doubleSay("hello")));
result //=> "Hello, hello!"
```

## Initial Implementation (would need updating to v7)

- Initial Spec PR by @littledan https://github.com/tc39/proposal-pipeline-operator/pull/51

## Old/Alternative Implementations

- Old PR to Babel: https://github.com/babel/babel/pull/3159
- https://github.com/SuperPaintman/babel-plugin-transform-pipeline
- `<<` and `>>`: https://github.com/michaelmitchell/babel-plugin-pipe-composition

cc @gilbert, @SuperPaintman

## Implementation

- [x] Parser PR: https://github.com/babel/babylon/pull/742
- [x] Transform PR: https://github.com/babel/babel/pull/6335
- Feedback around relating Pipeline Operator, Function Bind, Partial Application