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/babylon/src/plugins/flow.js

@@ -687,7 +687,9 @@ export default function (instance) {
   // export type
   instance.extend("shouldParseExportDeclaration", function (inner) {
     return function () {
-      return this.isContextual("type") || inner.call(this);
+      return this.isContextual("type")
+          || this.isContextual("interface")
+          || inner.call(this);
     };
   });
 
@@ -745,6 +747,11 @@ export default function (instance) {
           // export type Foo = Bar;
           return this.flowParseTypeAlias(declarationNode);
         }
+      } else if (this.isContextual("interface")) {
+        node.exportKind = "type";
+        let declarationNode = this.startNode();
+        this.next();
+        return this.flowParseInterface(declarationNode);
       } else {
         return inner.call(this, node);
       }