Fix output file

[bertsky]

I don't know why I am the first to notice, but this is what you would get when using the -o / --output_filename option:

Undefined subroutine &DTA::TEI::Text::Transform::write_file called at /perl5/lib/perl5/DTA/TEI/Text/Transform.pm line 157.

In this fix I assume what is meant was File::Slurp. (Perhaps in a past version of perl this was implicit?)

The second commit ensures that stdout is only written if using -v / --to_stdout (as opposed to always, which defies providing the option in the first place, and does not make sense if you want to just use -o).