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Fixes: #63

Add 2 new config options to control special files in templates using
unix-style file name patterns:

* templates.ignore_files to ignore/skip files entirely
* templates.plain_copy_files to copy files without rendering

Docs and tests are included.

Fixes: #58

Support Jinja include directive:

    {% include 'distro/common/foo' %}

Paths are relative to project root.

A simple project demonstrating apkg templating features.