Templates: add a way to attach a file, 'attach_output' or 'A', file whose absolute...

Templates: add a way to attach a file, 'attach_output' or 'A', file whose absolute path+filename
is the output of a command-line (it s a mix of 'attach' and 'program'). The first line of the
command-line output is taken into account, trailing newline being stripped down.
Like 'attach', if the filename is not found or not accessible, CM will raise an error. Like
'program', if the command-line fails, no error raises but the standard output of CM shows it.