add gdefault option

lib/ex-mode.coffee

@@ -50,3 +50,8 @@ module.exports = ExMode =
       description: 'when splitting, split from right'
       type: 'boolean'
       default: 'false'
+    gdefault:
+      title: 'Gdefault'
+      description: 'When on, the ":substitute" flag \'g\' is default on'
+      type: 'boolean'
+      default: 'false'

lib/vim-option.coffee

@@ -56,4 +56,10 @@ class VimOption
   noscs: =>
     @nosmartcase()
 
+  gdefault: =>
+    atom.config.set("ex-mode.gdefault", true)
+
+  nogdefault: =>
+    atom.config.set("ex-mode.gdefault", false)
+
 module.exports = VimOption