fix(types): envPrefix should be optional (#305)

Author: bcoe

lib/yargs-parser-types.ts

@@ -98,7 +98,7 @@ export interface Options {
   /** Provide default values for keys: `{ default: { x: 33, y: 'hello world!' } }`. */
   default: Dictionary<any>;
   /** Environment variables (`process.env`) with the prefix provided should be parsed. */
-  envPrefix: string;
+  envPrefix?: string;
   /** Specify that a key requires n arguments: `{ narg: {x: 2} }`. */
   narg: Dictionary<number>;
   /** `path.normalize()` will be applied to values set to this key. */