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-plugin-transform-flow-comments/test/fixtures/flow-comments/export-type-alias/actual.js

@@ -5,4 +5,6 @@ export type GraphQLFormattedError = {
     line: number,
     column: number
   }>
-};
+};
+export interface foo { p: number }
+export interface foo<T> { p: T }

packages/babel-plugin-transform-flow-comments/test/fixtures/flow-comments/export-type-alias/expected.js

@@ -6,3 +6,5 @@
     column: number
   }>
 };*/
+/*:: export interface foo { p: number }*/
+/*:: export interface foo<T> { p: T }*/