Add htmltotext extractor

Saves HTML text nodes and selected element attributes in
`htmltotext.txt` for each Snapshot. Primarily intended to be used
for search indexing.

archivebox/index/html.py

@@ -143,7 +143,7 @@ def snapshot_icons(snapshot) -> str:
             "mercury": "🅼",
             "warc": "📦"
         }
-        exclude = ["favicon", "title", "headers", "archive_org"]
+        exclude = ["favicon", "title", "headers", "htmltotext", "archive_org"]
         # Missing specific entry for WARC
 
         extractor_outputs = defaultdict(lambda: None)

archivebox/index/schema.py

@@ -429,6 +429,7 @@ class Link:
             'singlefile_path': 'singlefile.html',
             'readability_path': 'readability/content.html',
             'mercury_path': 'mercury/content.html',
+            'htmltotext_path': 'htmltotext.txt',
             'pdf_path': 'output.pdf',
             'screenshot_path': 'screenshot.png',
             'dom_path': 'output.html',
@@ -452,6 +453,7 @@ class Link:
                 'singlefile_path': static_path,
                 'readability_path': static_path,
                 'mercury_path': static_path,
+                'htmltotext_path': static_path,
             })
         return canonical