add layout.transition attribute

... which will be used to set transition options during
    Plotly.react updates.

Author: etpinard

src/plots/animation_attributes.js

@@ -69,6 +69,7 @@ module.exports = {
             role: 'info',
             min: 0,
             dflt: 500,
+            editType: 'none',
             description: [
                 'The duration of the transition, in milliseconds. If equal to zero,',
                 'updates are synchronous.'
@@ -116,6 +117,7 @@ module.exports = {
                 'bounce-in-out'
             ],
             role: 'info',
+            editType: 'none',
             description: 'The easing function used for the transition'
         },
     }

src/plots/layout_attributes.js

@@ -9,7 +9,9 @@
 'use strict';
 
 var fontAttrs = require('./font_attributes');
+var animationAttrs = require('./animation_attributes');
 var colorAttrs = require('../components/color/attributes');
+var extendFlat = require('../lib/extend').extendFlat;
 
 var globalFont = fontAttrs({
     editType: 'calc',
@@ -253,5 +255,12 @@ module.exports = {
             description: 'Sets the color of the active or hovered on icons in the modebar.'
         },
         editType: 'modebar'
-    }
+    },
+
+    transition: extendFlat({}, animationAttrs.transition, {
+        description: [
+            'Sets transition options used during Plotly.react updates.'
+        ].join(' '),
+        editType: 'none'
+    })
 };