Configuration Reference

[Mischback]

While working on the configuration reference (as tracked in #23) it feels like duplicating too much.

configs/postfix/main.cf does already include lots of documentation, it feels unnecessary to duplicate this again for the Sphinx-based documentation.

Instead, let's parse the actual configuration files, extract the comments and generate the Sphinx documentation automatically.

For reference: 9f08744 has rST compatible documentation of some Postfix settings.