Fix broken season number in episode (Fixes #73, #74)

Apparently, omdb did not specify the Season number in an Episode object.
Checking more recent data, it looks like it does. Now that we have it, we can
rely on it rather than forcing the constructor to take an episode number.

src/imdb.ts

@@ -249,9 +249,19 @@ export class Episode extends Movie {
      *
      * @throws TypeError when the episode number is invalid
      */
-    constructor(obj: OmdbEpisode, season: number) {
+    constructor(obj: OmdbEpisode, season?: number) {
         super(obj);
-        this.season = season;
+
+
+        if (season !== undefined) {
+            this.season = season
+        } else {
+            this.season = parseInt(obj.Season, 10);
+            if (isNaN(this.season)) {
+                throw new TypeError("invalid season");
+            }
+        }
+
         if (obj.hasOwnProperty("Episode")) {
             this.episode = parseInt(obj.Episode, 10);
             if (isNaN(this.episode)) {
@@ -584,7 +594,7 @@ export class Client {
             } else if (isTvshow(data)) {
                 ret = new TVShow(data, opts);
             } else if (isEpisode(data)) {
-                ret = new Episode(data, 30);
+                ret = new Episode(data);
             } else {
                 throw new ImdbError(`type: '${data.Type}' is not valid`);
             }

src/interfaces.ts

@@ -49,6 +49,7 @@ export interface OmdbTvshow {
 export interface OmdbEpisode {
     Title: string;
     Released: string;
+    Season: string;
     Episode: string;
     Type: string;
     imdbRating: string;