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,
     }
 }