make layout meta a valType:'any'

- to start accepting `meta: {key: value}` settings

Author: etpinard

Diff for: src/plots/layout_attributes.js

@@ -410,15 +410,18 @@ module.exports = {
     },
 
     meta: {
-        valType: 'data_array',
+        valType: 'any',
+        arrayOk: true,
         editType: 'plot',
         description: [
             'Assigns extra meta information that can be used in various `text` attributes.',
             'Attributes such as the graph, axis and colorbar `title.text`, annotation `text`',
             '`trace.name` in legend items, `rangeselector`, `updatemenues` and `sliders` `label` text',
             'all support `meta`. One can access `meta` fields using template strings:',
             '`%{meta[i]}` where `i` is the index of the `meta`',
-            'item in question.'
+            'item in question.',
+            '`meta` can also be an object for example `{key: value}` which can be accessed',
+            '%{meta[key]}.'
         ].join(' ')
     },