Chip Input: Should split pasted text

#### Bug, feature request, or proposal:
Bug

#### What is the expected behavior?
Pasting the text: `apple, banana, orange,` into a [chip input](https://material.angular.io/components/chips/overview#chip-input) should result in three different chips.

#### What is the current behavior?
Produces a single chip with all three items.

#### What are the steps to reproduce?
1. Copy the text: `apple, banana, orange,`
1. Paste it into the example input on the Angular Material chip [input demo site](https://material.angular.io/components/chips/overview#chip-input)
1. Press `Enter`
1. There *should* be three chips (right now, only there is only one)

#### What is the use-case or motivation for changing an existing behavior?
Allow inputting data via copy/paste.

#### Which versions of Angular, Material, OS, TypeScript, browsers are affected?
Angular Material 6.2.1

#### Is there anything else we should know?

Angular Material のチップ入力デモにあるチップ入力の例から始め、`apple, banana, orange,` の貼り付けを再現します。Enter が押されたときに貼り付けられた値がどのように処理されるかを追跡し、完成した動作によって 1 つではなく 3 つの個別のチップが生成されることを確認します。

索引モデルが issue の本文から書いたものです。