Redefined subarray in index.d.ts to return Buffer (#298)

Author: BTOdell

index.d.ts

@@ -7,6 +7,7 @@ export class Buffer extends Uint8Array {
     compare(otherBuffer: Uint8Array, targetStart?: number, targetEnd?: number, sourceStart?: number, sourceEnd?: number): number;
     copy(targetBuffer: Buffer, targetStart?: number, sourceStart?: number, sourceEnd?: number): number;
     slice(start?: number, end?: number): Buffer;
+    subarray(start?: number, end?: number): Buffer;
     writeUIntLE(value: number, offset: number, byteLength: number, noAssert?: boolean): number;
     writeUIntBE(value: number, offset: number, byteLength: number, noAssert?: boolean): number;
     writeIntLE(value: number, offset: number, byteLength: number, noAssert?: boolean): number;