initial support for child extractor options

Using "parent-category>child-category" as extractor category in a config
file allows to set options for a child extractor when it was spawned by
that parent.

For example "reddit>gfycat" to set gfycat options for when it was found
in a reddit post.

{
    "extractor": {
        "gfycat": {
            "filename": "regular filename"
        },
        "reddit>gfycat": {
            "filename": "reddit-specific filename"
        }
    }
}

Note: This does currently not work for most imgur links due to how its
extractor hierarchy is structured.

gallery_dl/extractor/common.py

@@ -45,10 +45,6 @@ class Extractor():
     def __init__(self, match):
         self.log = logging.getLogger(self.category)
         self.url = match.string
-
-        if self.basecategory:
-            self.config = self._config_shared
-            self.config_accumulate = self._config_shared_accumulate
         self._cfgpath = ("extractor", self.category, self.subcategory)
         self._parentdir = ""
 
@@ -98,16 +94,22 @@ class Extractor():
         return config.accumulate(self._cfgpath, key)
 
     def _config_shared(self, key, default=None):
-        return config.interpolate_common(("extractor",), (
+        return config.interpolate_common(
-            (self.category, self.subcategory),
+            ("extractor",), self._cfgpath, key, default)
-            (self.basecategory, self.subcategory),
-        ), key, default)
 
     def _config_shared_accumulate(self, key):
-        values = config.accumulate(self._cfgpath, key)
+        first = True
-        conf = config.get(("extractor",), self.basecategory)
+        extr = ("extractor",)
+
+        for path in self._cfgpath:
+            if first:
+                first = False
+                values = config.accumulate(extr + path, key)
+            else:
+                conf = config.get(extr, path[0])
                 if conf:
-            values[:0] = config.accumulate((self.subcategory,), key, conf=conf)
+                    values[:0] = config.accumulate(
+                        (self.subcategory,), key, conf=conf)
         return values
 
     def request(self, url, method="GET", session=None,

gallery_dl/job.py

@@ -32,6 +32,21 @@ class Job():
         self.kwdict = {}
         self.status = 0
 
+        cfgpath = []
+        if parent and parent.extractor.category != extr.category:
+            cat = "{}>{}".format(
+                parent.extractor.category, extr.category)
+            cfgpath.append((cat, extr.subcategory))
+            cfgpath.append((extr.category, extr.subcategory))
+        if extr.basecategory:
+            if not cfgpath:
+                cfgpath.append((extr.category, extr.subcategory))
+            cfgpath.append((extr.basecategory, extr.subcategory))
+        if cfgpath:
+            extr._cfgpath = cfgpath
+            extr.config = extr._config_shared
+            extr.config_accumulate = extr._config_shared_accumulate
+
         actions = extr.config("actions")
         if actions:
             from .actions import parse