Missing function to strip multiline metadata annotations

[`stripAnnotations`](https://github.com/aerogear/graphql-metadata/blob/master/src/annotations/stripAnnotations.ts) can remove any line that begins with `@`, but will not strip multiline metadata annotations like this:

```graphql
"""
@model(
  create: true,
  update: false
)
"""
type User {
}
```

A `stripMetadata` function is needed for this.
