generalize tag-splitting option (#92)

- extend functionality to other booru sites:
  - http://behoimi.org/
  - https://konachan.com/
  - https://e621.net/
  - https://rule34.xxx/
  - https://safebooru.org/
  - https://yande.re/

gallery_dl/extractor/safebooru.py

@@ -17,6 +17,7 @@ class SafebooruExtractor(booru.XmlParserMixin,
     """Base class for safebooru extractors"""
     category = "safebooru"
     api_url = "https://safebooru.org/index.php"
+    post_url = "https://safebooru.org/index.php?page=post&s=view&id={}"
 
     def __init__(self, match):
         super().__init__(match)
@@ -27,7 +28,7 @@ class SafebooruTagExtractor(booru.TagMixin, SafebooruExtractor):
     """Extractor for images from safebooru.org based on search-tags"""
     pattern = [(r"(?:https?://)?(?:www\.)?safebooru\.org/(?:index\.php)?"
                 r"\?page=post&s=list&tags=(?P<tags>[^&#]+)")]
-    test = [("http://safebooru.org/index.php?page=post&s=list&tags=bonocho", {
+    test = [("https://safebooru.org/index.php?page=post&s=list&tags=bonocho", {
         "url": "17c61b386530cf4c30842c9f580d15ef1cd09586",
         "content": "e5ad4c5bf241b1def154958535bef6c2f6b733eb",
     })]
@@ -37,7 +38,14 @@ class SafebooruPostExtractor(booru.PostMixin, SafebooruExtractor):
     """Extractor for single images from safebooru.org"""
     pattern = [(r"(?:https?://)?(?:www\.)?safebooru\.org/(?:index\.php)?"
                 r"\?page=post&s=view&id=(?P<post>\d+)")]
-    test = [("http://safebooru.org/index.php?page=post&s=view&id=1169132", {
+    test = [("https://safebooru.org/index.php?page=post&s=view&id=1169132", {
         "url": "cf05e37a3c62b2d55788e2080b8eabedb00f999b",
         "content": "93b293b27dabd198afafabbaf87c49863ac82f27",
+        "options": (("tags", True),),
+        "keyword": {
+            "tags_artist": "kawanakajima",
+            "tags_character": "heath_ledger ronald_mcdonald the_joker",
+            "tags_copyright": "dc_comics mcdonald's the_dark_knight",
+            "tags_general": str,
+        },
     })]