Change type of the all property in the constructor (#73)

This commit corrects the type of the `all` property in the `mitt` constructor function, such that it properly reflects a list of `Handler` functions rather than a single `Handler` function.

This makes it possible to provide an argument to the `all` property in TypeScript projects without casting the argument to the `any` type and with proper error feedback.

Author: jesperzach

mitt.d.ts

@@ -9,7 +9,7 @@ declare namespace mitt {
 	type WildcardHandler = (type?: string, event?: any) => void;
 
 	interface MittStatic {
-		(all?: {[key: string]: Handler}): Emitter;
+		(all?: {[key: string]: Array<Handler>}): Emitter;
 	}
 
 	interface Emitter {