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?

Start with the chip input example on the Angular Material chip input demo and reproduce the paste of `apple, banana, orange,`. Trace how the pasted value is handled when Enter is pressed, and verify that the completed behavior produces three separate chips instead of one.

Written by the indexing model from the issue text.