Clear button flag.

New prop, 'clearable': Can the user clear the selected property?

Usage, :clearable="false" (defaults to true.)

See issue #463.

src/components/Select.vue

@@ -423,6 +423,15 @@
         default: false
       },
 
+      /**
+       * Can the user clear the selected property?
+       * @type {Boolean}
+       */
+      clearable: {
+        type: Boolean,
+        default: true
+      },
+
       /**
        * Sets the max-height property on the dropdown list.
        * @deprecated
@@ -1053,7 +1062,7 @@
        * @return {Boolean}
        */
       showClearButton() {
-        return !this.multiple && !this.open && this.mutableValue != null
+        return !this.multiple && this.clearable && !this.open && this.mutableValue != null
       }
     },