extend 'path-restrict' option

Allow its value to be a JSON object / Python dict that specifies
a mapping from invalid/unwanted input characters to specific
output characters.

For example {"/": "-", "*": "+"} will transform
"foo / ***bar***" into "foo - +++bar+++"

(closes #662, #755)

gallery_dl/util.py

@@ -690,6 +690,9 @@ class PathFormat():
     def _build_cleanfunc(chars, repl):
         if not chars:
             return lambda x: x
+        elif isinstance(chars, dict):
+            def func(x, table=str.maketrans(chars)):
+                return x.translate(table)
         elif len(chars) == 1:
             def func(x, c=chars, r=repl):
                 return x.replace(c, r)