Add %I for identifiers padded with spaces

It calculates if actual todo id is shorter than 3 characters and adds
additional space or two to maintain fixed length of this parameter.

Also set %I as default instead of %i to provide compatibility with
tests and.

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| %c %{(}p{)} %s',
             },
 
             'tags': {

topydo/lib/ListFormat.py

+# Topydo - A todo.txt client written in Python.
+# Copyright (C) 2014 - 2015 Bram Schoenmakers <me@bramschoenmakers.nl>
+#
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+""" Ulities for formatting output with "list_format" option."""
+
+def filler(p_str, p_len):
+    """
+    Returns p_str preceded by additional spaces if p_str is shorter than p_len.
+    """
+    to_fill = p_len - len(p_str)
+    return to_fill*' ' + p_str

topydo/lib/PrettyPrinterFilter.py

@@ -22,6 +22,7 @@ from six import u
 
 from topydo.lib.Colors import NEUTRAL_COLOR, Colors
 from topydo.lib.Config import config
+from topydo.lib.ListFormat import filler
 
 
 class PrettyPrinterFilter(object):
@@ -150,6 +151,9 @@ class PrettyPrinterFormatFilter(PrettyPrinterFilter):
             # todo ID
             'i': lambda t: str(self.todolist.number(t)),
 
+            # todo ID pre-filled with 1 or 2 spaces if its length is <3
+            'I': lambda t: filler(str(self.todolist.number(t)), 3),
+
             # list of tags (spaces)
             'K': lambda t: ' '.join(['{}:{}'.format(tag, value)
                                      for tag, value in sorted(p_todo.tags())]),