add option to write logging output to a file

- '--write-log FILE' as cmdline argument
- 'output.logfile' as config file option

docs/configuration.rst

@@ -72,6 +72,7 @@ Description Controls whether the output strings should be shortened to fit
             on one console line.
 =========== =====
 
+
 output.progress
 ---------------
 =========== =====
@@ -89,6 +90,15 @@ Description Controls the progress indicator when *gallery-dl* is run with
 =========== =====
 
 
+output.logfile
+--------------
+=========== =====
+Type        ``string``
+Default     ``null``
+Description File to write logging output to.
+=========== =====
+
+
 Downloader Options
 ==================
 

docs/gallery-dl.conf

@@ -127,6 +127,7 @@
     {
         "mode": "auto",
         "shorten": true,
-        "progress": true
+        "progress": true,
+        "logfile": null,
     }
 }

gallery_dl/__init__.py

@@ -21,7 +21,7 @@ if sys.hexversion < 0x3030000:
     sys.exit(1)
 
 import logging
-from . import version, config, option, extractor, job, exception
+from . import version, config, option, extractor, job, util, exception
 
 __version__ = version.__version__
 log = logging.getLogger("gallery-dl")
@@ -108,6 +108,21 @@ def main():
             except AttributeError:
                 pass
 
+        # logfile
+        logfile = config.interpolate(("output", "logfile"))
+        if logfile:
+            try:
+                handler = logging.FileHandler(util.expand_path(logfile))
+            except OSError as exc:
+                log.warning("logfile: %s", exc)
+            else:
+                formatter = logging.Formatter(
+                    "[%(asctime)s][%(name)s][%(levelname)s] %(message)s",
+                    "%Y-%m-%d %H:%M:%S")
+                handler.setFormatter(formatter)
+                handler.setLevel(logging.INFO)
+                logging.getLogger().addHandler(handler)
+
         if args.list_modules:
             for module_name in extractor.modules:
                 print(module_name)

gallery_dl/option.py

@@ -93,12 +93,6 @@ def build_parser():
         metavar="FILE", action=ConfigAction, dest="cookies",
         help="File to load additional cookies from",
     )
-    general.add_argument(
-        "--write-unsupported",
-        metavar="FILE", dest="unsupportedfile",
-        help=("Write URLs, which get emitted by other extractors but cannot "
-              "be handled, to FILE"),
-    )
 
     output = parser.add_argument_group("Output Options")
     output.add_argument(
@@ -133,6 +127,17 @@ def build_parser():
         help=("Print a list of extractor classes "
               "with description, (sub)category and example URL"),
     )
+    output.add_argument(
+        "--write-log",
+        metavar="FILE", dest="logfile", action=ConfigAction,
+        help=("Write logging output to FILE"),
+    )
+    output.add_argument(
+        "--write-unsupported",
+        metavar="FILE", dest="unsupportedfile",
+        help=("Write URLs, which get emitted by other extractors but cannot "
+              "be handled, to FILE"),
+    )
 
     downloader = parser.add_argument_group("Downloader Options")
     downloader.add_argument(