Feature request: add an option to limit the number of items user can select in multi select

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

#### What is the expected behavior?
In multi select, add an option to limit the number of items user can select in multi select

#### What is the current behavior?
User can select unlimited number of items in multi select

#### What is the use-case or motivation for changing an existing behavior?
It is not uncommon to see questions in surveys like: you may choose up to 3 reasons why you like our product

#### Which versions of Angular, Material, OS, TypeScript, browsers are affected?
Angular Material 2.0.0-beta.8

Start by locating Angular Material's multi-select component and its existing tests; read how selection changes are handled and how component options are exposed. Done means a configurable selection limit works for multi-select while preserving normal selection behavior, with tests covering the limit.

Written by the indexing model from the issue text.