Made keepEmptyLines optional

If one decides to only use something of `TransformOptions` (e.g. `encoding`), it wouldn't be possible to omit the `keepEmptyLines` option.

byline/byline.d.ts

@@ -9,7 +9,7 @@ declare module "byline" {
     import stream = require("stream");
 
     export interface LineStreamOptions extends stream.TransformOptions {
-        keepEmptyLines: boolean;
+        keepEmptyLines?: boolean;
     }
 
     export interface LineStream extends stream.Transform {
@@ -35,4 +35,4 @@ declare module "byline" {
     export function createStream(stream:NodeJS.ReadableStream, options?:LineStreamOptions):LineStream;
 
     export var LineStream:LineStreamCreatable;
-}
+}