Use parameter for dynamic name if arg is not provided

[revati]

Can use validation-name to set dynamic field name

<validator name="validation1">
  <input v-validate="['required']" :validation-name="fieldName">
  <span v-show="$validation1[fieldName].required">Required your name.</span>
</validator>

This can fix #46

[kazupon]

Thank you for pull request.

I've published the now.
https://github.com/vuejs/vue-validator/releases/tag/v2.0.0-alpha.11

Please try it. 😸