[deepmerge] Allow partial types in deepmerge

Sometimes deepmerge is used for overwriting only a few properties in a larger object. Allowing partial types lets users deepmerge without having to typecast in those cases.
In addition, also updated the typings for arrayMerge to indicate that it will be operating on two arrays.

types/deepmerge/deepmerge-tests.ts

@@ -16,4 +16,14 @@ const expected = {
   quux: 5
 };
 
-const result = deepmerge<any>(x, y);
+const result = deepmerge(x, y);
+const anyResult = deepmerge<any>(x, y);
+
+function reverseConcat(dest: number[], src: number[]) {
+    return src.concat(dest);
+}
+
+const withOptions = deepmerge(x, y, {
+  clone: false,
+  arrayMerge: reverseConcat
+});

types/deepmerge/index.d.ts

@@ -1,17 +1,19 @@
 // Type definitions for deepmerge 1.3
 // Project: https://github.com/KyleAMathews/deepmerge
 // Definitions by: marvinscharle <https://github.com/marvinscharle>
+//                 syy1125 <https://github.com/syy1125>
 // Definitions: https://github.com/DefinitelyTyped/DefinitelyTyped
 
 export = deepmerge;
 
-declare function deepmerge<T>(x: T, y: T, options?: deepmerge.Options<T>): T;
+declare function deepmerge<T>(x: Partial<T>, y: Partial<T>, options?: deepmerge.Options): T;
+declare function deepmerge<T1, T2>(x: T1, y: T2, options?: deepmerge.Options): T1 & T2;
 
 declare namespace deepmerge {
-    interface Options<T> {
+    interface Options {
         clone?: boolean;
-        arrayMerge?(destination: T, source: T, options?: Options<T>): T;
+        arrayMerge?(destination: any[], source: any[], options?: Options): any[];
     }
 
-    function all<T>(objects: T[], options?: Options<T>): T;
+    function all<T>(objects: Array<Partial<T>>, options?: Options): T;
 }