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/rule34.py

@@ -17,6 +17,7 @@ class Rule34Extractor(booru.XmlParserMixin,
     """Base class for rule34 extractors"""
     category = "rule34"
     api_url = "https://rule34.xxx/index.php"
+    post_url = "https://rule34.xxx/index.php?page=post&s=view&id={}"
     page_limit = 4000
 
     def __init__(self, match):
@@ -28,7 +29,7 @@ class Rule34TagExtractor(booru.TagMixin, Rule34Extractor):
     """Extractor for images from rule34.xxx based on search-tags"""
     pattern = [(r"(?:https?://)?(?:www\.)?rule34\.xxx/(?:index\.php)?"
                 r"\?page=post&s=list&tags=(?P<tags>[^&#]+)")]
-    test = [("http://rule34.xxx/index.php?page=post&s=list&tags=danraku", {
+    test = [("https://rule34.xxx/index.php?page=post&s=list&tags=danraku", {
         "content": "a01768c6f86f32eb7ebbdeb87c30b0d9968d7f97",
         "pattern": r"https?://(.?img\.)?rule34\.xxx/images/\d+/[0-9a-f]+\.jpg",
         "count": 2,
@@ -39,6 +40,14 @@ class Rule34PostExtractor(booru.PostMixin, Rule34Extractor):
     """Extractor for single images from rule34.xxx"""
     pattern = [(r"(?:https?://)?(?:www\.)?rule34\.xxx/(?:index\.php)?"
                 r"\?page=post&s=view&id=(?P<post>\d+)")]
-    test = [("http://rule34.xxx/index.php?page=post&s=view&id=1974854", {
+    test = [("https://rule34.xxx/index.php?page=post&s=view&id=1974854", {
         "content": "fd2820df78fb937532da0a46f7af6cefc4dc94be",
+        "options": (("tags", True),),
+        "keyword": {
+            "tags_artist": "danraku",
+            "tags_character": "io_(pso2)",
+            "tags_copyright": "phantasy_star phantasy_star_online_2",
+            "tags_general": "blue_hair female",
+            "tags_metadata": "absurdres highres",
+        },
     })]