Add recoll engine (#2325)

recoll is a local search engine based on Xapian:
http://www.lesbonscomptes.com/recoll/

By itself recoll does not offer web or API access,
this can be achieved using recoll-webui:
https://framagit.org/medoc92/recollwebui.git

This engine uses a custom 'files' result template

set `base_url` to the location where recoll-webui can be reached
set `dl_prefix` to a location where the file hierarchy as indexed by recoll can be reached
set `search_dir` to the part of the indexed file hierarchy to be searched, use an empty string to search the entire search domain

searx/settings.yml

@@ -650,6 +650,28 @@ engines:
     shortcut : qws
     categories : social media
 
+#  - name: library
+#    engine: recoll
+#    shortcut: lib
+#    base_url: 'https://recoll.example.org/'
+#    search_dir: ''
+#    mount_prefix: /export
+#    dl_prefix: 'https://download.example.org'
+#    timeout: 30.0
+#    categories: files
+#    disabled: True
+
+#  - name: recoll library reference
+#    engine: recoll
+#    base_url: 'https://recoll.example.org/'
+#    search_dir: reference
+#    mount_prefix: /export
+#    dl_prefix: 'https://download.example.org'
+#    shortcut: libr
+#    timeout: 30.0
+#    categories: files
+#    disabled: True
+
   - name : reddit
     engine : reddit
     shortcut : re