Only match a placeholder when both [] brackets are present.

Suppose format string:

 ls_format = %s [%T]

The result would be:

 Test todo item [just now

The final bracket is missing, because it was matched as part of the
placeholder.

This change will only match the closing bracket when the opening bracket
is also present.

topydo/lib/PrettyPrinterFilter.py

@@ -200,10 +200,10 @@ class PrettyPrinterFormatFilter(PrettyPrinterFilter):
             repl = getter(p_todo)
             pattern = (r'(?P<start>.*)'
                        r'%(?P<before>{{.+?}})?'
-                       r'\[?(?P<placeholder>{})\]?'
+                       r'(?P<placeholder>{ph}|\[{ph}\])'
                        r'(?P<after>{{.+?}})?'
                        r'(?P<whitespace>\s)*'
-                       r'(?P<end>.*)').format(placeholder)
+                       r'(?P<end>.*)').format(ph=placeholder)
             match = re.match(pattern, p_todo_str)
             if match:
                 if repl == '':