[pp:metadata] add 'newline' option (#8439)

* Add configuration setting to control newline characters in metadata.
* update docs/configuration
* introduce 'open()' method
* add 'newline' test

gallery_dl/postprocessor/metadata.py

@@ -117,9 +117,15 @@ class MetadataPP(PostProcessor):
         self.mtime = options.get("mtime")
         self.omode = options.get("open", omode)
         self.encoding = options.get("encoding", "utf-8")
+        self.newline = options.get("newline")
         self.skip = options.get("skip", False)
         self.meta_path = options.get("metadata-path")
 
+    def open(self, path):
+        return open(path, self.omode,
+                    encoding=self.encoding,
+                    newline=self.newline)
+
     def run(self, pathfmt):
         archive = self.archive
         if archive and archive.check(pathfmt.kwdict):
@@ -138,11 +144,11 @@ class MetadataPP(PostProcessor):
             return
 
         try:
-            with open(path, self.omode, encoding=self.encoding) as fp:
+            with self.open(path) as fp:
                 self.write(fp, pathfmt.kwdict)
         except FileNotFoundError:
             os.makedirs(directory, exist_ok=True)
-            with open(path, self.omode, encoding=self.encoding) as fp:
+            with self.open(path) as fp:
                 self.write(fp, pathfmt.kwdict)
 
         if archive: