Template instantiation removes comments from zkg.meta

[ckreibich]

The template instantiation process will currently read an existing zkg.meta via a configparser.ConfigParser, update it with template metadata, and then write it out again. Config parsers don't preserve comments, so these get dropped in the resulting package. I'm not sure this is a problem, but it is unintuitive. zkg could instead do a sanity check whether the existing zkg.meta has no template and template_vars sections (they shouldn't), and append them like in any text file.