- Add pico-theme.yml with a theme's API version, theme-specific default Twig config, registering theme-specific custom meta headers and defaults for Pico's `theme_config` config
- Add new `onThemeLoading(&$theme)` and `onThemeLoaded($theme, $themeApiVersion, &$themeConfig)` events
- Enable Twig autoescaping by default
Instead of using `*.config.php` files, use `*.yml` files to configure Pico. YAML is much easier to understand, more user friendly and (at least a bit) more error-tolerant, but still very powerful. Don't break BC by letting `PicoDeprecated` still read `config/config.php`.
I always thought that doing this is pretty unusual... But now it simply breaks BC - please refer to @Lomanic's [comment](https://github.com/picocms/Pico/pull/260#issuecomment-152610857). Using a return statement has no advantages, but increases the probability that something goes wrong (e.g. a clueless user removes the return statement). It was introduced with 23b90e2, but we never released it ([v0.9.1](4cb2b24fae/lib/pico.php (L188-L189))). Removing the return statement shouldn't cause any problems even for users which installed Pico in the meantime. As a result we don't break BC and moreover remove a prior BC break 😃
