[Sort] Implement API to clear sort

#### Bug, feature request, or proposal:
I would like to know if there is a provision available to clear the sorting set by user on a mat-table column dynamically.

#### What is the expected behavior?
Provision to clear sort order set by user on mat-table

#### What is the current behavior?
Once user applies sort on a particular column, it is maintained permanently.

#### What are the steps to reproduce?
Click on one column header of a mat-table with sorting implemented. The table will be sorted based on the column data. Now if we need to reset the sorting, we need to click on the same column header 2 more times.

#### What is the use-case or motivation for changing an existing behavior?
Our requirement is to reset the user selected sort order on every reload.

#### Which versions of Angular, Material, OS, TypeScript, browsers are affected?
Angular CLI: 1.7.3
Node: 8.9.4
OS: win32 x64
Angular: 5.2.9

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

mat-table のソートのエントリポイントから開始し、ユーザーが選択したソート状態がどのように維持されるかを追跡します。要求されたリセット動作に沿って clear-sorting API を定義し、その後、ヘッダーを繰り返しクリックしなくてもテーブルを未ソート状態に戻せ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。