Add sort fields for projects and contexts

 topydo ls -s project

to print all todo items grouped by project.

topydo/lib/Sorter.py

@@ -38,6 +38,8 @@ def get_field_function(p_field):
     if is_priority_field(p_field):
         # assign dummy priority when a todo has no priority
         result = lambda a: a.priority() or 'ZZ'
+    elif p_field == 'context' or p_field == 'contexts':
+        result = lambda a: sorted([c.lower() for c in a.contexts()])
     elif p_field == 'creationdate' or p_field == 'creation':
         # when a task has no creation date, push it to the end by assigning it
         # the maximum possible date.
@@ -50,6 +52,8 @@ def get_field_function(p_field):
         result = importance
     elif p_field == 'importance-avg' or p_field == 'importance-average':
         result = average_importance
+    elif p_field == 'project' or p_field == 'projects':
+        result = lambda a: sorted([c.lower() for c in a.projects()])
     elif p_field == 'text':
         result = lambda a: a.text()
     else: