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!

Reproduziere das Problem mit den gezeigten index.js- und types.js-Beispielen unter Verwendung von documentation.js und untersuche anschließend, wie importierte Module und Flow-Typimporte behandelt werden. Bestätige die Verhaltensänderung, indem du prüfst, dass die generierte Dokumentation den Typ MyNumber enthält, ohne einen separaten Wertimport hinzuzufügen.

Vom Indexierungsmodell aus dem Issue-Text verfasst.