[feat] engines: add dogpile (general, news, images, videos)

Add support for the Dogpile search engine, found at:

https://seirdy.one/posts/2021/03/10/search-engines-with-own-indexes/

It seems to use the same index as startpage because results are similar and they
share the ``qadf`` (Safe-Search) request parameter.

searx/settings.yml

@@ -803,6 +803,34 @@ engines:
     display_type: ["infobox"]
     categories: [general]
 
+  - name: dogpile
+    engine: dogpile
+    shortcut: dog
+    dogpile_categ: search
+    categories: general
+    disabled: true
+
+  - name: dogpile images
+    engine: dogpile
+    shortcut: dogi
+    dogpile_categ: images
+    categories: images
+    disabled: true
+
+  - name: dogpile videos
+    engine: dogpile
+    shortcut: dogv
+    dogpile_categ: videos
+    categories: videos
+    disabled: true
+
+  - name: dogpile news
+    engine: dogpile
+    shortcut: dogn
+    dogpile_categ: news
+    categories: news
+    disabled: true
+
   # duckduckgo uses html.duckduckgo.com,
   # duckduckgo web uses duckduckgo.com
   - name: duckduckgo