Adds a comic format registry

There is the limitation that you still need to manually import all modules that register a new comic format, but this does give us a lot of flexibility

comic_download/xkcd.py

@@ -16,15 +16,20 @@ import requests
 from PIL import Image, ImageDraw, ImageFont
 from bs4 import BeautifulSoup
 
-from comic_download.comic_strip import Comic, ComicStrip
+from comic_download.comic_strip import Comic, ComicStrip, register_comic_class
 
 class XKCDComic(Comic):
     """
     The XKCD Comic.
     """
+    
     def __init__(self):
         super().__init__("xkcd")
 
+    @classmethod
+    def create_from_url(cls, url):
+        return cls()
+
     def _load_data(self):
         self.title = "XKCD"
         self.author = "Randal Munroe"
@@ -37,6 +42,15 @@ class XKCDComic(Comic):
         self.data = result.json()
         self.latest_identifier = self.data["num"]
 
+    def get_first_strip(self):
+        return 1
+
+    def get_latest_strip(self):
+        return self.latest_identifier
+
+    def get_all_strips(self) -> list:
+        return list(range(self.get_first_strip(), self.get_latest_strip() + 1))
+
     def __new__(cls):
         if not hasattr(cls, 'instance'):
             cls.instance = super(XKCDComic, cls).__new__(cls)
@@ -46,13 +60,14 @@ class XKCDComicStrip(ComicStrip):
     """
     A comic strip object represents a single XKCD strip.
     """
-    def __init__(self, index: int):
+    def __init__(self, comic, index: int):
         """
         Creates a comic strip object.
 
+        :param comic: The XKCD comic.
         :param index: The index of the strip to load.
         """
-        super().__init__(XKCDComic(), index)
+        super().__init__(comic, index)
         self.url = f"https://xkcd.com/{index}"
         self.data_url = f"{self.url}/info.0.json"
         self.transcript = None
@@ -120,3 +135,7 @@ class XKCDComicStrip(ComicStrip):
                 f_img.save(f_image)
                 f_image.close()
                 self.transformed_images.append(f_image)
+
+
+XKCDComic.strip_cls = XKCDComicStrip
+register_comic_class("xkcd.com", XKCDComic)