[formatter] implement 'D' format specifier

To be able to parse any string into a 'datetime' object
and format it as necessary.

Example:

{created_at:D%Y-%m-%dT%H:%M:%S%z}
->
"2010-01-01 00:00:00"

{created_at:D%Y-%m-%dT%H:%M:%S%z/%b %d %Y %I:%M %p}
->
"Jan 01 2010 12:00 AM"

with 'created_at' == "2010-01-01T01:00:00+0100"

gallery_dl/formatter.py

@@ -274,6 +274,8 @@ def build_format_func(format_spec):
             return _parse_join(format_spec)
         if fmt == "R":
             return _parse_replace(format_spec)
+        if fmt == "D":
+            return _parse_datetime(format_spec)
         return _default_format(format_spec)
     return format
 
@@ -319,6 +321,16 @@ def _parse_replace(format_spec):
     return replace
 
 
+def _parse_datetime(format_spec):
+    dt_format, _, format_spec = format_spec.partition("/")
+    dt_format = dt_format[1:]
+    fmt = build_format_func(format_spec)
+
+    def dt(obj):
+        return fmt(text.parse_datetime(obj, dt_format))
+    return dt
+
+
 def _default_format(format_spec):
     def wrap(obj):
         return format(obj, format_spec)