Update docs with information about callables (#486)

Add note in documentation about callables and `$request` and `$response`
being provided as arguments.

In addition this adds two anchors to the documentation and changes the
name of the event listener anchor to be consistent with the other anchors.

docs/develop/event_listeners.rst

@@ -62,7 +62,7 @@ When writing an event listener for Imbo you can choose one of the following appr
 Below you will find examples on the approaches mentioned above.
 
 .. note::
-    Information regarding how to **attach** the event listeners to Imbo is available in the :ref:`event listener configuration <configuration-event-listeners>` section.
+    Information regarding how to **attach** the event listeners to Imbo is available in the :ref:`event listener configuration <event-listeners-configuration>` section.
 
 Implement the ``Imbo\EventListener\ListenerInterface`` interface
 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
@@ -149,7 +149,7 @@ For testing and/or debugging purposes you can also write the event listener dire
         // ...
     ];
 
-The ``$event`` object passed to the function is the same as in the previous two examples. This approach should mostly be used for testing purposes and quick hacks. More information regarding this approach is available in the :ref:`event listener configuration <configuration-event-listeners>` section.
+The ``$event`` object passed to the function is the same as in the previous two examples. This approach should mostly be used for testing purposes and quick hacks. More information regarding this approach is available in the :ref:`event listener configuration <event-listeners-configuration>` section.
 
 Subscribing to events using a wild card
 +++++++++++++++++++++++++++++++++++++++

docs/installation/configuration.rst

@@ -11,6 +11,16 @@ The configuration file(s) you need to create should simply return arrays with co
     :local:
     :depth: 1
 
+.. _configuration-with-callables:
+
+Using callables in configuration
+--------------------------------
+
+Imbo supports providing callables for certain configuration options. In Imbo 2 these are the :ref:`accessControl <access-control-configuration>`, :ref:`database <database-configuration>`, :ref:`eventListeners[name] <event-listeners-configuration>`, :ref:`resource <resource-configuration>` and :ref:`storage <storage-configuration>` options, while Imbo 3 adds callable support for :ref:`transformationPresets <transformation-presets-configuration>`. The callable receives two arguments (`$request` and `$response`) which map to the active request and response objects.
+
+By using a callable you can extend Imbo with custom logic for most configuration options (or provide the ``$request`` or ``$response`` objects to your implementing class), such as switching storage modules based on which user performed the request.
+
+
 .. _access-control-configuration:
 
 Imbo access control - ``accessControl``
@@ -710,7 +720,7 @@ Example usage:
         // ...
     ];
 
-.. _configuration-event-listeners:
+.. _event-listeners-configuration:
 
 Event listeners - ``eventListeners``
 ------------------------------------
@@ -995,6 +1005,8 @@ Imbo itself includes an event listener initializer in the default configuration
 
 .. note:: Only event listeners specified as strings (class names) in the configuration will be instantiated by Imbo, so event listeners instantiated in the configuration array, either directly or via a Closures, will not be initialized by the configured event listener initializers.
 
+.. _transformation-presets-configuration:
+
 Image transformation presets - ``transformationPresets``
 --------------------------------------------------------
 
@@ -1043,6 +1055,8 @@ By doing this the ``thumbnail`` part of the ``fixedGraythumb`` preset will ignor
 
 .. note:: The URLs will stay the same if you change the transformation chain in a preset. Keep this in mind if you use for instance Varnish or some other HTTP accelerator in front of your web server(s).
 
+.. _resource-configuration:
+
 Custom resources and routes - ``resources`` and ``routes``
 ----------------------------------------------------------