[feature request] Add ack-like output format?

[Leo Liu]

I don't know which program first started using this format. But ack¹ has
it. In this format file names aren't repeated for each line instead
things are grouped if they are from the same file. See this sample:

--------------------------------
Dancer/Response.pm
120:sub push_header {
126:            $self->{headers}->push_header( $header => $h );
178:        $self->push_header(

Dancer.pm
86:  push_header
164:sub push_header     { Dancer::SharedData->response->push_header(@_); }
411:            $resp->push_header('Content-Disposition' => 
891:see L</push_header>.
893:=head2 push_header
898:        push_header 'x-my-header' => '1';
899:        push_header 'x-my-header' => '2';
......
--------------------------------

Footnotes: 
¹  http://beyondgrep.com