Implement `file_append' task action

lib/producer/core/actions/file_append.rb

@@ -0,0 +1,23 @@
+module Producer
+  module Core
+    module Actions
+      class FileAppend < Action
+        def apply
+          fs.file_write path, combined_content
+        end
+
+        def path
+          arguments[0]
+        end
+
+        def content
+          arguments[1]
+        end
+
+        def combined_content
+          fs.file_read(path) + content
+        end
+      end
+    end
+  end
+end