Flow type imports syntax is not enough to have module documented

Hi,

I'm trying to use `documentation.js` 4.0.0-beta.18 with `Flow` and it's working pretty well for infering types. However, my code is spread across several modules and I would like to use the [Flow type imports syntax](https://flowtype.org/docs/modules.html#type-imports--exports) like this:

```javascript
// index.js
// @flow
import type {MyNumber} from './types';

/**
* My number
*/
const myNumber: MyNumber = 1;
```

```javascript
// types.js
// @flow

/**
* My number type
*/
type MyNumber = number;

export type {
MyNumber
};
```

Here is the page generated by `documentation.js`:


But instead I would expect:


Where `MyNumber` type is added to the documentation.

If I change `index.js` like this:

```diff
// @flow
+import './types';
import type {MyNumber} from './types';
```

Then I'll have the result of the second image.

So to me, it seems like an `import type` statement is not enough to have `documentation.js` look up the content of the imported module.

Can we add support for this `import type` syntax into `documentation.js` ? If yes, do you have an idea of where should I start if I wanted to change the actual behaviour and submit a PR ?

Thanks!

示されている index.js と types.js の例を documentation.js で使用して問題を再現し、その後、インポートされたモジュールと Flow の型インポートがどのように扱われるかを調査してください。生成されたドキュメントに MyNumber 型が含まれ、別個の値のインポートを追加する必要がないことを確認して、動作の変更を確認してください。

索引モデルが issue の本文から書いたものです。