(optionally) define configuration in config.php file (outside VCS)

To improve Wikitten's integration with a VCS, a `config.php` file can optionally be used to define app configuration. This file is kept out of the repository, making it possible for a user to customize an installation without coming across conflicts when running updates.

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### Usage:

1. Copy `config.php.example` over to `config.php`

    $ cp config.php.example config.php

2. Edit it to your liking
3. That's it!