implement string slicing for format strings

It is now possible to slice string (or list) values of format string
replacement fields with the same syntax as in regular Python code.

"{digits}"       -> "0123456789"
"{digits[2:-2]}" -> "234567"
"{digits[:5]}"   -> "01234"

The optional third parameter (step) has been left out to simplify things.

gallery_dl/util.py

@@ -344,6 +344,10 @@ class Formatter():
         for is_attr, i in rest:
             if is_attr:
                 obj = getattr(obj, i)
+            elif ":" in i:
+                start, _, stop = i.partition(":")
+                start = int(start) if start else 0
+                return obj[start:int(stop)] if stop else obj[start:]
             else:
                 obj = obj[i]