Enhancement - Custom report limit override

- If a custom report has a specified limit to the number of tasks
  it shows, then that limit may be  overridden by specifying a new
  quantity on the command line.
- Added unit tests to verify expected behavior.
- Updated documentation to reflect new functionality.
- Removed obsolete documentation references to an "oldest" and
  "newest" configuration variable.

src/task.cpp

@@ -133,7 +133,7 @@ static std::string shortUsage (Config& conf)
   table.addCell (row, 2, "Removes the 'start' time from a task");
 
   row = table.addRow ();
-  table.addCell (row, 1, "task done ID");
+  table.addCell (row, 1, "task done ID [tags] [attrs] [desc...]");
   table.addCell (row, 2, "Marks the specified task as completed");
 
   row = table.addRow ();