Feature #800

- Added feature #800, adding a new command 'columns' that lists all the columns
  available for custom reports, and includes their formatting options (thanks
  to T. Charles Yun).

src/columns/ColDue.cpp

@@ -42,6 +42,12 @@ ColumnDue::ColumnDue ()
 {
   _name      = "due";
   _label     = STRING_COLUMN_LABEL_DUE;
+
+  _styles.push_back ("countdown");
+
+  Date now;
+  now += 125;
+  _examples.push_back (Duration (now - Date ()).formatCompact ());
 }
 
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