feat(rating): allow custom star templates

[maxokorokov]

Part of #801

Adds a template to allow custom rating display
Default rendering still uses ★ and ☆ symbols

<template #t let-fill="fill">
  <!-- fill is a number between 0-100 to represent how the star is filled -->
</template>

<ngb-rating [(rate)]="currentRate" [starTemplate]="t"></ngb-rating>

[wesleycho]

LGTM

[maxokorokov]

Rebased

[pkozlowski-opensource]

This LGTM as well. What we could / should do in a separate PR is to allow star templates as child elements of the ngb-rating:

<ngb-rating ...>
    <template>...</template>
</ngb-rating>

This is the "trick" that ngFor is using so it would be cool to support this as well. But we can totally do this in a separate PR.