add 'd' conversion for format strings

to convert a timestamp to a formattable 'datetime' object.

For example '{created_at!d:%Y-%m-%d}'
transforms the timestamp in 'created_at' into a 'datetime' object
and then formats its content using '%Y-%m-%d' as template.

1262304000 -> datetime(2010, 1, 1) -> "2010-01-01"

gallery_dl/util.py

@@ -506,6 +506,7 @@ class Formatter():
     - "c": calls str.capitalize
     - "C": calls string.capwords
     - "t": calls str.strip
+    - "d": calls text.parse_timestamp
     - "U": calls urllib.parse.unquote
     - "S": calls util.to_string()
     - Example: {f!l} -> "example"; {f!u} -> "EXAMPLE"
@@ -537,6 +538,7 @@ class Formatter():
         "c": str.capitalize,
         "C": string.capwords,
         "t": str.strip,
+        "d": text.parse_timestamp,
         "U": urllib.parse.unquote,
         "S": to_string,
         "s": str,