implement logging options

Standard logging to stderr, logfiles, and unsupported URL files (which
are now handled through the logging module) can now be configured by
setting their respective option keys (log, logfile, unsupportedfile)
to a dict and specifying the following options;

- format:
    format string for logging messages
    available keys: see [1]
    default: "[{name}][{levelname}] {message}"
- format-date:
    format string for {asctime} fields in logging messages
    available keys: see [2]
    default: "%Y-%m-%d %H:%M:%S"
- level:
    the lowercase levelname until which the logger should activate;
    available levels are debug, info, warning, error, exception
    default: "info"
- path:
    path of the file to be written to
- mode:
    'mode' argument when opening the specified file
    can be either "w" to truncate the file or "a" to append to it (see [3])

If 'output.log', '.logfile', or '.unsupportedfile' is a string, it will
be interpreted, as it has been, as the filepath
(or as format string for .log)

[1] https://docs.python.org/3/library/logging.html#logrecord-attributes
[2] https://docs.python.org/3/library/time.html#time.strftime
[3] https://docs.python.org/3/library/functions.html#open

docs/gallery-dl-example.conf

@@ -100,7 +100,21 @@
     "output":
     {
         "mode": "terminal",
-        "logfile": "~/gallery-dl/log.txt"
+        "log": {
+            "format": "{name}: {message}",
+            "level": "info"
+        },
+        "logfile": {
+            "path": "~/gallery-dl/log.txt",
+            "mode": "w",
+            "level": "debug"
+        },
+        "unsupportedfile": {
+            "path": "~/gallery-dl/unsupported.txt",
+            "mode": "a",
+            "format": "{asctime} {message}",
+            "format-date": "%Y-%m-%d-%H-%M-%S"
+        }
     },
 
     "cache": {