Add CLI mode to use the Kirby CMS API from a PHP script

With this additions, you can include kirby.php instead of system.php when you want to use the Kirby API and manually fetch pages.

This (or something similar) is required for kpm to create a static HTML version of a Kirby installation - which actually works in the moment and is available in the kpm repo.

~~There are some bugs left - for example `$page->isOpen()` and `$page->isActive()` don't work yet because of depending on $_SERVER stuff.
But basic stuff actually works completely.~~

EDIT: Works perfectly with [that commit](https://github.com/vis7mac/kirbycms/commit/415dbb1103f298ac47268dafe8b1bc915eecbcf7).