Support passing Element into loading container property

[OzairP]

Summary

This PR allows passing Element as the container property for the first parameter of Vue.prototype.$vs.loading

Rationale

Refs provide a direct link to the DOM that your component uses, using ID's in Vue is dangerous since components can be spun up and multiple ID's can exist.

Using the Vue ref api we can pass the element itself into the loader to yield a more precise and predictable behavior

Backwards Compatibility

This change is backwards compatible.

Example

<template>
    <vs-button class="vs-con-loading__container" vs-color="primary" vs-type="filled" @click="login">Login</vs-button>
</template>

<script lang="ts">
    import Vue from 'vue'
    import { Component } from 'vue-property-decorator'

    @Component()
    export default class Login extends Vue {
        public login () {
            this.$vs.loading({
                container: (this.$refs.login as Vue).$el,
                background: 'primary',
                color: '#FFFFFF',
                scale: 0.45
            })
        }
    }
</script>