Add support for GLUE v2

#31 was an attempt to provide support for fixing https://github.com/quattor/template-library-grid/issues/45. Unfortunately it is not really appropriate and there is a simple solution. This is an attempt to implement it. It is backward compatible with pre #31 version. Despite it is an alternative, the code introduced by #31 is currently kept and will be removed when https://github.com/quattor/template-library-grid/issues/45 is fixed. #31 approach is problematic in that it allows potentially abritrary command execution...

Would be good to have it in 14.10 to allow fixing the long standing https://github.com/quattor/template-library-grid/issues/45 issue...
euhm, how about
```perl
my $attrstxt = join(", ", @{$attrs->{$key}});
$attrstxt = "($attrstxt)" if @{$attrs->{$key}} > 1;
$contents .= "$key = $attrstxt\n";
```