Use "conditional characters" in list_format

These conditional characters should be specified **inside** curly braces
({}) and **after** percent sign (%). They can appear in two groups:
before and after particular placeholder.

Example:
"list_format = %{(}p{)}"

If priority of given todo is C, "list_format" set above will now resolve
to (C), but also won't display empty parentheses if there is no priority
set.

Fixes issue mentioned in 8650033b

topydo.conf

@@ -15,7 +15,7 @@ auto_creation_date          = 1
 hide_tags                   = id,p,ical
 indent                      = 0
 list_limit                  = -1
-list_format                 = |%i| (%p) %s
+list_format                 = |%i| %{(}p{)} %s
 
 [tags]
 tag_start                   = t

topydo/lib/Config.py

@@ -69,7 +69,7 @@ class _Config:
                 'hide_tags': 'id,p,ical',
                 'indent': '0',
                 'list_limit': '-1',
-                'list_format': '|%i| (%p) %s',
+                'list_format': '|%i| %{(}p{)} %s',
             },
 
             'tags': {

topydo/lib/PrettyPrinterFilter.py

@@ -173,7 +173,37 @@ class PrettyPrinterFormatFilter(PrettyPrinterFilter):
         p_todo_str = self.format
 
         for placeholder, getter in placeholders.items():
-            p_todo_str = re.sub(r'%\[?{}\]?'.format(placeholder), getter(p_todo), p_todo_str)
+            repl = getter(p_todo)
+            pattern = (r'(?P<start>.*)'
+                       r'%(?P<before>{{.+?}})?'
+                       r'\[?(?P<placeholder>{})\]?'
+                       r'(?P<after>{{.+?}})?'
+                       r'(?P<whitespace>\s)*'
+                       r'(?P<end>.*)').format(placeholder)
+            if repl == '':
+                p_todo_str = re.sub(pattern, match.group('start') + match.group('end'), p_todo_str)
+            else:
+                def strip_braces(p_matchobj):
+                    try:
+                        before = p_matchobj.group('before').strip('{}')
+                    except AttributeError:
+                        before = ''
+
+                    placeholder = p_matchobj.group('placeholder')
+
+                    try:
+                        after = p_matchobj.group('after').strip('{}')
+                    except AttributeError:
+                        after = ''
+
+                    whitespace = p_matchobj.group('whitespace') or ''
+                    start = p_matchobj.group('start') or ''
+                    end = p_matchobj.group('end') or ''
+
+                    return start + before + '%' + placeholder + after + whitespace + end
+
+                p_todo_str = re.sub(pattern, strip_braces, p_todo_str)
+                p_todo_str = re.sub(r'%{}'.format(placeholder), repl, p_todo_str)
 
         return p_todo_str