[formatter] implement 'M' format specifier

Map a list of objects to a list of values

[
    {"name": "John Doe"      , "age": 42},
    {"name": "Jane Smith"    , "age": 24},
    {"name": "Max Mustermann", "age": null}
]

== :Mname =>

["John Doe", "Jane Smith", "Max Mustermann"]

gallery_dl/formatter.py

@@ -221,7 +221,7 @@ class FStringFormatter():
     """Generate text by evaluating an f-string literal"""
 
     def __init__(self, fstring, default=NONE, fmt=None):
-        self.format_map = util.compile_expression('f"""' + fstring + '"""')
+        self.format_map = util.compile_expression(f'f"""{fstring}"""')
 
 
 class TemplateFormatter(StringFormatter):
@@ -302,7 +302,7 @@ def _parse_optional(format_spec, default):
     fmt = _build_format_func(format_spec, default)
 
     def optional(obj):
-        return before + fmt(obj) + after if obj else ""
+        return f"{before}{fmt(obj)}{after}" if obj else ""
     return optional
 
 
@@ -385,6 +385,27 @@ def _parse_join(format_spec, default):
     return apply_join
 
 
+def _parse_map(format_spec, default):
+    key, _, format_spec = format_spec.partition(_SEPARATOR)
+    key = key[1:]
+    fmt = _build_format_func(format_spec, default)
+
+    def map_(obj):
+        if not obj or isinstance(obj, str):
+            return fmt(obj)
+
+        results = []
+        for item in obj:
+            if isinstance(item, dict):
+                value = item.get(key, ...)
+                results.append(default if value is ... else value)
+            else:
+                results.append(item)
+        return fmt(results)
+
+    return map_
+
+
 def _parse_replace(format_spec, default):
     old, new, format_spec = format_spec.split(_SEPARATOR, 2)
     old = old[1:]
@@ -507,6 +528,7 @@ _FORMAT_SPECIFIERS = {
     "D": _parse_datetime,
     "J": _parse_join,
     "L": _parse_maxlen,
+    "M": _parse_map,
     "O": _parse_offset,
     "R": _parse_replace,
     "S": _parse_sort,