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

test/bug.634.t

@@ -45,7 +45,7 @@ if (open my $fh, '>', 'bug.rc')
 qx{../src/task rc:bug.rc add Test};
 
 # Result: Attempt to undo add with confirmation=off
-my $output = qx{echo 'n' |../src/task rc:bug.rc rc.confirmation=off undo};
+my $output = qx{echo '-- n' |../src/task rc:bug.rc rc.confirmation=off undo};
 unlike ($output, qr/Are you sure/ms, 'Undo honours confirmation=off.');
 
 # Cleanup.