add 'j' format string conversion

to convert to a JSON formatted string

docs/formatting.md

@@ -62,6 +62,12 @@ Conversion specifiers allow to *convert* the value to a different form or type.
     <td><code>{foo!C}</code></td>
     <td><code>Foo Bar</code></td>
 </tr>
+<tr>
+    <td align="center"><code>j</code></td>
+    <td>Serialize value to a JSON formatted string</td>
+    <td><code>{tags!j}</code></td>
+    <td><code>["sun", "tree", "water"]</code></td>
+</tr>
 <tr>
     <td align="center"><code>t</code></td>
     <td>Trim a string, i.e. remove leading and trailing whitespace characters</td>

gallery_dl/util.py

@@ -547,6 +547,7 @@ class Formatter():
     - "u": calls str.upper
     - "c": calls str.capitalize
     - "C": calls string.capwords
+    - "j". calls json.dumps
     - "t": calls str.strip
     - "d": calls text.parse_timestamp
     - "U": calls urllib.parse.unquote
@@ -581,6 +582,7 @@ class Formatter():
         "u": str.upper,
         "c": str.capitalize,
         "C": string.capwords,
+        "j": json.dumps,
         "t": str.strip,
         "T": to_timestamp,
         "d": text.parse_timestamp,

test/test_util.py

@@ -297,6 +297,7 @@ def test_conversions(self):
         self._run_test("{t!d}", datetime.datetime(2010, 1, 1))
         self._run_test("{t!d:%Y-%m-%d}", "2010-01-01")
         self._run_test("{dt!T}", "1262304000")
+        self._run_test("{l!j}", '["a", "b", "c"]')
 
         with self.assertRaises(KeyError):
             self._run_test("{a!q}", "hello world")