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?

Beginne mit dem Chip-Input-Beispiel in der Angular Material Chip-Input-Demo und reproduziere das Einfügen von `apple, banana, orange,`. Verfolge, wie der eingefügte Wert verarbeitet wird, wenn Enter gedrückt wird, und überprüfe, dass das abgeschlossene Verhalten drei separate Chips statt eines erzeugt.

Vom Indexierungsmodell aus dem Issue-Text verfasst.