Add option to clone submodules recursively

[zmughal]

One reason why this plugin may be needed is that GitHub archive links do not contain submodules at this time.

A work around for now is to do the following:

use File::chdir;

share {
  ...
  plugin 'Download::Git';
  # Need to fetch the submodules too.
  meta->around_hook( fetch => sub {
    my $orig  = shift;
    my $build = shift;
    my $data  = $orig->($build, @_);

    if( $data->{type} eq 'file' ) {
      local $CWD = $data->{path};
      $build->log("In $CWD");
      $build->system(
        '%{git}',
        qw(submodule update),
        qw(--init --recursive),
      );
    }

    return $data;
  });
}

[plicease]

Seems reasonable.