Add field type extensions

This change enables to extend fields with custom types
in order to be able to inject custom body in a field wrapper.

Author: saleyn

config/config.exs

@@ -1,3 +1,5 @@
 import Config
 
 config :phoenix, :json_library, Jason
+
+import_config "#{config_env()}.exs"

config/test.exs

@@ -0,0 +1,4 @@
+import Config
+
+config :wallaby, :chromedriver,
+  binary: System.get_env("CHROMEDRIVER", "/usr/bin/chromedriver")

lib/petal_components/behaviors/field_extension.ex

@@ -0,0 +1,31 @@
+defmodule PetalComponents.Field.Extension do
+  @moduledoc """
+  Behavior for field extensions.
+  """
+
+  @doc """
+  This callback function takes assigns and returns a rendered field's body.
+
+  ## Example
+      @behaviour PetalComponents.Field.Extension
+
+      def render(assigns) do
+        ~H'''
+        <div>Custom Field</div>
+        '''
+      end
+  """
+  @callback render(assigns :: map) :: map
+
+  @doc """
+  This callback function is used to get classes for the custom field.
+
+  ## Example
+      @behaviour PetalComponents.Field.Extension
+
+      def get_class_for_type("pretty_field"), do: "pc-text-input"
+  """
+  @callback get_class_for_type(String.t) :: String.t
+
+  @optional_callbacks get_class_for_type: 1
+end