Allow async extension functions

According to http://jestjs.io/docs/en/expect.html#expectextendmatchers

> expect.extends also supports async matchers. Async matchers return a Promise so you will need to await the returned value. Let's use an example matcher to illustrate the usage of them. We are going to implement a very similar matcher than toBeDivisibleBy, only difference is that the divisible number is going to be pulled from an external source.

This updates the typings of `ExpectExtendMap` to allow a `Promise` to be returned.

types/jest/index.d.ts

@@ -269,7 +269,7 @@ declare namespace jest {
     }
 
     interface ExpectExtendMap {
-        [key: string]: (this: MatcherUtils, received: any, ...actual: any[]) => { message(): string, pass: boolean };
+        [key: string]: (this: MatcherUtils, received: any, ...actual: any[]) => { message(): string, pass: boolean } | Promise<{ message(): string, pass: boolean }>;
     }
 
     interface SnapshotSerializerOptions {