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?

Commencez par l’exemple d’entrée de chips de la démo Angular Material et reproduisez le collage de `apple, banana, orange,`. Suivez la façon dont la valeur collée est traitée lorsque vous appuyez sur Enter, et vérifiez que le comportement final produit trois chips distincts au lieu d’un seul.

Rédigé par le modèle d'indexation à partir du texte de l'issue.