New 'ids' command, and stdin reading

- New 'ids' command that returns a filtered set of task ID numbers, instead
  of the actual tasks.  For advanced pipeline use.
- Now supplements the command line with data read from standard input, which
  allows commands like:  echo 'add Pay the bills' | task

src/report.cpp

@@ -244,6 +244,10 @@ int shortUsage (std::string& outs)
   table.addCell (row, 1, "task count [filter]");
   table.addCell (row, 2, "Shows only the number of matching tasks.");
 
+  row = table.addRow ();
+  table.addCell (row, 1, "task ids [filter]");
+  table.addCell (row, 2, "Shows only the IDs of matching tasks, in the form of a range.");
+
   row = table.addRow ();
   table.addCell (row, 1, "task version");
   table.addCell (row, 2, "Shows the task version number.");