refactor: plugin class

[logaretm]

This refactors the plugin to use a class to install itself and to hold its config and dependencies. It also allows using the plugin programmatically without installing it. Much like VueI18n.

This allows configuring vee-validate without having to call Vue.use. Which helps using i18n with the verify API.

import VeeValidate from 'vee-validate';

const vee = new VeeValidate({
  // config...
});

closes #1638