mat-icon: To be able to have both svgIcon and fontIcon

#### Please describe the feature you would like to request.
Let's say you have a js structure (an array of object) holding a list of icons, some svg, some font with other properties.
In your view you want to have a simple *ngFor to iterate on all your icons.
For svg icons, your object will have a "svg" property.
For font icons, your object will have a "font" property.
The key will be undefined if not set.
Therefore, you will have in your loop something like:
`{{ btn.icon }}`
Sounds pretty easy but that's not working right now (Angular Material 7.3.7).

#### What is the use-case or motivation for this proposal?
To simply KISS
To avoid a fugly:
``
`{{ btn.icon }}`

#### Is there anything else we should know?
For what I saw, the reason about that is located in:
https://github.com/angular/material2/blob/master/src/lib/icon/icon.ts#L224-L244

A quick hack could be :
https://github.com/angular/material2/blob/master/src/lib/icon/icon.ts#L237
`!this._usingFontIcon() && this._clearSvgElement();`

src/lib/icon/icon.ts の224-244行目から始め、報告で説明されている Angular Material 7.3.7 の mat-icon/ngFor の例で問題を再現します。svgIcon と fontIcon の処理を追跡し、その後、既存の動作を維持しながら、1つの mat-icon で条件付きテンプレートなしにどちらの形式もレンダリングでき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。