Private Properties Support

It would seem that private properties is not yet supported by this plugin.

## Works

```javascript
class PubChan {
#pipeline;
}
```

## Doesn't Work

```javascript
class PubChan {
#pipeline: {};
}
```


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**update** interestingly, it would appear the `React` eslint plugin adds support for this as I can add this on any react projects but not on my others using just flow & proposal-class-properties.