Add support for "declare interface" Flow syntax

This has been a feature in Flow for a long time (couldn't easily find a
specific commit adding this). Interfaces are basically undocumented, though, so
it's easy to see how this was missed.

packages/babel-generator/src/generators/flow.js

@@ -36,6 +36,11 @@ export function DeclareFunction(node: Object) {
   this.semicolon();
 }
 
+export function DeclareInterface(node: Object) {
+  this.push("declare ");
+  this.InterfaceDeclaration(node);
+}
+
 export function DeclareModule(node: Object) {
   this.push("declare module ");
   this.print(node.id, node);