Add optional="true" to fx argument includes. Fixes #191

Author: kswedberg

includes/complete-argument.xml

@@ -1,4 +1,4 @@
 <?xml version="1.0"?>
-<argument name="complete" type="Function">
+<argument name="complete" type="Function" optional="true">
 	<desc>A function to call once the animation is complete.</desc>
 </argument>

includes/duration-argument.xml

@@ -1,5 +1,5 @@
 <?xml version="1.0"?>
-<argument name="duration" default="400">
+<argument name="duration" default="400" optional="true">
 	<desc>A string or number determining how long the animation will run.</desc>
 	<type name="Number"/>
 	<type name="String"/>

includes/easing-argument.xml

@@ -1,4 +1,4 @@
 <?xml version="1.0"?>
-<argument name="easing" type="String" default="swing">
+<argument name="easing" type="String" default="swing" optional="true">
 	<desc>A string indicating which easing function to use for the transition.</desc>
 </argument>