Add export interface Flow syntax support

An interface export is just like a type export. In fact, it's a syntax affordance which makes the following equivalent:

```javascript
interface I_ { ... }
export type I = I_;
```

```javascript
export interface I { ... }
```

See facebook/flow#1145

packages/babel-generator/test/fixtures/flow/type-annotations/actual.js

@@ -98,3 +98,5 @@ import type, { foo } from "bar";
 import type * as namespace from "bar";
 export type { foo };
 export type { foo } from "bar";
+export interface foo { p: number };
+export interface foo<T> { p: T };

packages/babel-generator/test/fixtures/flow/type-annotations/expected.js

@@ -102,3 +102,5 @@ import type, { foo } from "bar";
 import type * as namespace from "bar";
 export type { foo };
 export type { foo } from "bar";
+export interface foo { p: number };
+export interface foo<T> { p: T };