Add API for overwriting default components

[sagalbot]

New

Adds a components function prop that allows user to implement their own components and override the defaults.

example

<template>
  <div>
    <v-select
        :options="['Canada', 'United States']"
        :components="defaults => ({...defaults, Deselect})"
    />
  </div>
</template>

<script>
import Vue from 'vue';

export default {
  name: 'ChildComponentExample',
  computed: {
    Deselect () {
      return Vue.component('Deselect', {
        render (createElement) {
          return createElement('button', 'Clear');
        },
      });
    },
  },
};
</script>

Result

Todos

• create dedicated templating docs
• add page for child components
• add page for SCSS variables
• implement higher level API to set child components when registering vSelect
• test coverage

[coveralls]

Coverage increased (+2.09%) to 96.392% when pulling 330ce44 on add-components-prop into d522aca on master.