add simple progress indicator for multiple URLs (#19)

The output can be configured via the 'output.progress'
config value.

Possible values:
    - true:     Show the default progress indicator
                "[{current}/{total}] {url}" (default)
    - false:    Never show the progress indicator
    - <string>: Show the progress indicator using this
                as a custom format string(1).
                Possible replacement keys are:
                - current: current URL index
                - total  : total number of URLs
                - url    : current URL

(1) https://docs.python.org/3/library/string.html#formatstrings

docs/gallery-dl.conf

@@ -59,5 +59,11 @@
             "date-max": 253402210800,
             "recursion": 0
         }
+    },
+    "output":
+    {
+        "mode": "auto",
+        "shorten": true,
+        "progress": true
     }
 }