Parameter Decorators (Stage 0)

Original issue submitted by @armorik83 in https://github.com/babel/babel/issues/1301

Champion: @igorminar
Spec Repo: N/A
Spec Doc: https://docs.google.com/document/d/1Qpkqf_8NzAwfD8LdnqPjXAQ2wwh8BBUGynhn-ZlCWT0/edit

## Example

```js
class MyComponent {
refresh(@lastRefreshTime timeStamp) {}
}
```

- [ ] I would wait on regular decorators support to start this (https://github.com/babel/proposals/issues/11)

Start with the linked regular decorators proposal and the linked spec document; the issue names no repository files or tests. Establish the implementation scope and acceptance criteria for parameter decorators before considering the work complete.

Written by the indexing model from the issue text.