implement extractor.add() and .add_module()

... as a public and non-hacky way to add (external) extractors to
gallery-dl's pool and make them available for extractor.find()

gallery_dl/extractor/__init__.py

@@ -103,6 +103,23 @@ def find(url):
     return None
 
 
+def add(klass):
+    """Add 'klass' to the list of available extractors"""
+    for pattern in klass:
+        _cache.append((re.compile(pattern), klass))
+
+
+def add_module(module):
+    """Add all extractors in 'module' to the list of available extractors"""
+    tuples = [
+        (re.compile(pattern), klass)
+        for klass in _get_classes(module)
+        for pattern in klass.pattern
+    ]
+    _cache.extend(tuples)
+    return tuples
+
+
 def extractors():
     """Yield all available extractor classes"""
     return sorted(
@@ -139,14 +156,9 @@ def _list_patterns():
     yield from _cache
 
     for module_name in _module_iter:
-        module = importlib.import_module("."+module_name, __package__)
-        tuples = [
-            (re.compile(pattern), klass)
-            for klass in _get_classes(module)
-            for pattern in klass.pattern
-        ]
-        _cache.extend(tuples)
-        yield from tuples
+        yield from add_module(
+            importlib.import_module("."+module_name, __package__)
+        )
 
 
 def _get_classes(module):

gallery_dl/extractor/powermanga.py

@@ -26,6 +26,6 @@ class PowermangaMangaExtractor(foolslide.FoolslideMangaExtractor):
     category = "powermanga"
     pattern = foolslide.manga_pattern(r"read\.powermanga\.org")
     test = [("https://read.powermanga.org/series/one_piece/", {
-        "url": "6ba226780a3c1c1f1cc5f4a4b96c18260f4ec0f3",
-        "keyword": "576109177b1bb59ab2f55450cc9ef4a31e28714c",
+        "url": "3b2037a9ffe30ea0da4e710a40863f0693f21afe",
+        "keyword": "e2a924b0924cba711e78b3585ad24a97dec70006",
     })]

gallery_dl/extractor/rule34.py

@@ -30,7 +30,7 @@ class Rule34TagExtractor(booru.TagMixin, Rule34Extractor):
                 r"\?page=post&s=list&tags=(?P<tags>[^&#]+)")]
     test = [("http://rule34.xxx/index.php?page=post&s=list&tags=danraku", {
         "content": "a01768c6f86f32eb7ebbdeb87c30b0d9968d7f97",
-        "pattern": r"https?://b?img\.rule34\.xxx/images/\d+/[0-9a-f]+\.jpg",
+        "pattern": r"https?://(.?img\.)?rule34\.xxx/images/\d+/[0-9a-f]+\.jpg",
         "count": 2,
     })]