improve config lookup when there are multiple possible locations

This specifically applies to all Mastodon extractors and all
extractors with a 'basecategory', i.e. 'booru', 'foolslide', etc.

Values inside those general config locations wouldn't be recognized
when a value with the same was set on the 'extractor' level.

For example 'extractor.mastodon.directory' should be used over
'extractor.directory' when both are set, but this was impossible
with the previous implementation.

(fixes #843)

gallery_dl/config.py

@@ -108,6 +108,38 @@ def interpolate(path, key, default=None, *, conf=_config):
     return default
 
 
+def interpolate_common(common, paths, key, default=None, *, conf=_config):
+    """Interpolate the value of 'key'
+    using multiple 'paths' along a 'common' ancestor
+    """
+    if key in conf:
+        return conf[key]
+
+    # follow the common path
+    try:
+        for p in common:
+            conf = conf[p]
+            if key in conf:
+                default = conf[key]
+    except Exception:
+        return default
+
+    # try all paths until a value is found
+    value = util.SENTINEL
+    for path in paths:
+        c = conf
+        try:
+            for p in path:
+                c = c[p]
+                if key in c:
+                    value = c[key]
+        except Exception:
+            pass
+        if value is not util.SENTINEL:
+            return value
+    return default
+
+
 def set(path, key, value, *, conf=_config):
     """Set the value of property 'key' for this session"""
     for p in path: