支持checkbox的X转换

src/components/Checkbox.vue

@@ -19,10 +19,13 @@ export default {
   computed: {
     model: {
       get () {
-        return this.value
+        if(this.value)
+          return true
+        else 
+          return false
       },
       set (newVal) {
-        this.$emit('input', newVal)
+        this.$emit('input', newVal?"X":"")
       }
     },
     mode () {