extend 'cookies-update' functionality

Allow writing cookies to a different file than a given cookies.txt,
making it possible to export cookies imported with --cookies-from-browser

To convert browser cookies to cookies.txt format:
  gallery-dl --cookies-fr chromium \
             -o cookies-update=cookies.txt \
             --no-download \
             http://example.org/file.jpg

docs/configuration.rst

@@ -457,13 +457,17 @@ Description
 extractor.*.cookies-update
 --------------------------
 Type
-    ``bool``
+    * ``bool``
+    * |Path|_
 Default
     ``true``
 Description
-    If `extractor.*.cookies`_ specifies the |Path|_ of a cookies.txt
+    Export session cookies in cookies.txt format.
-    file and it can be opened and parsed without errors,
+
-    update its contents with cookies received during data extraction.
+    * If this is a |Path|_, write cookies to the given file path.
+
+    * If this is ``true`` and `extractor.*.cookies`_ specifies the |Path|_
+      of a valid cookies.txt file, update its contents.
 
 
 extractor.*.proxy

gallery_dl/extractor/common.py

@@ -379,12 +379,22 @@ class Extractor():
 
     def _store_cookies(self):
         """Store the session's cookiejar in a cookies.txt file"""
-        if self._cookiefile and self.config("cookies-update", True):
+        export = self.config("cookies-update", True)
-            try:
+        if not export:
-                with open(self._cookiefile, "w") as fp:
+            return
-                    util.cookiestxt_store(fp, self._cookiejar)
+
-            except OSError as exc:
+        if isinstance(export, str):
-                self.log.warning("cookies: %s", exc)
+            path = util.expand_path(export)
+        else:
+            path = self._cookiefile
+            if not path:
+                return
+
+        try:
+            with open(path, "w") as fp:
+                util.cookiestxt_store(fp, self._cookiejar)
+        except OSError as exc:
+            self.log.warning("cookies: %s", exc)
 
     def _update_cookies(self, cookies, domain=""):
         """Update the session's cookiejar with 'cookies'"""