Add support for Laravel's configuration component

I wrote an Smarty plugin to support Laravel's configuration component.

To get a configuration value, the following syntax is used:

    {config key="app.timezone"}

One can also use a default value:

    {config key="app.timezone" default="UTC"}

The value can be passed to a template variable using the `assign` parameter:

    {config key="app.timezone" default="UTC" assign="timezone"}
    ⋮
    {$timezone}