fixes placeholder shown instead of value issue (#95)

Fixes the following issue:
#65

Author: sorcerdon

src/vue-numeric.vue

@@ -266,8 +266,8 @@ export default {
   },
 
   mounted () {
-    // Set default value props when placeholder undefined.
-    if (!this.placeholder) {
+    // Set default value props when valueNumber has some value 
+    if (this.valueNumber) {
       this.process(this.valueNumber)
       this.amount = this.format(this.valueNumber)