site_config should be separated from app.yml and have own entity functions automatically

[hkoba]

Main purpose of app.yml is to configure YATT itself. For site-wide, site-author-defined constants such as site_name and site_author, I have used site_config entry of app.yml. But mixing them into single config file is not useful for general Web development. (Who wants to gather application specific constants into php.ini?)

So, site_config should be separated to its own file like app.site_config.yml (or just site_config.yml if former doesn't exist).

Also since main purpose of having above site_config.yml is to help site authors, providing entity functions too for each site_config entry is natural yatt-way, isn't it?.