add 't' format string conversion (closes #1065)

to Trim whitespace from the beginning and end of strings.
Example: '{field!t}' becomes 'foo' for 'field' == "  \nfoo\t\r"

gallery_dl/util.py

@@ -495,6 +495,7 @@ class Formatter():
     - "u": calls str.upper
     - "c": calls str.capitalize
     - "C": calls string.capwords
+    - "t": calls str.strip
     - "U": calls urllib.parse.unquote
     - "S": calls util.to_string()
     - Example: {f!l} -> "example"; {f!u} -> "EXAMPLE"
@@ -525,6 +526,7 @@ class Formatter():
         "u": str.upper,
         "c": str.capitalize,
         "C": string.capwords,
+        "t": str.strip,
         "U": urllib.parse.unquote,
         "S": to_string,
         "s": str,