[formatter] support putting keys in quotes

i.e. obj["key"] or obj['key']
as in f-strings

gallery_dl/formatter.py

@@ -256,6 +256,8 @@ def parse_field_name(field_name):
             try:
                 if ":" in key:
                     key = _slice(key)
+                else:
+                    key = key.strip("\"'")
             except TypeError:
                 pass  # key is an integer
 

test/test_formatter.py

@@ -128,6 +128,11 @@ class TestFormatter(unittest.TestCase):
         self._run_test("{l[0]}" , "a")
         self._run_test("{a[6]}" , "w")
 
+    def test_dict_access(self):
+        self._run_test("{d[a]}"  , "foo")
+        self._run_test("{d['a']}", "foo")
+        self._run_test('{d["a"]}', "foo")
+
     def test_slicing(self):
         v = self.kwdict["a"]
         self._run_test("{a[1:10]}"  , v[1:10])