ionic generate modal

# Feature Request

#### Describe the Feature Request
I think it would be great to generate a modal. It should add `Modal` suffix instead of `Component` for the modal class name. Plus, in case it's an angular app, it would add the modal to `entryComponents`.

#### Describe Preferred Solution
```
ionic generate modal MODAL_PATH
```

#### Describe Alternatives
1. `ionic g c my-modal`
2. refactor `MyModalComponent` to `MyModal`
3. add `MyModal` to `app.module.ts > entryComponents`

#### Concerns:
It would be a problem to do `ionic g m my-modal` since `m` is for module.

首先定位現有的元件生成 schematic 以及 `ionic generate` 的命令註冊。追蹤 Angular 應用程式的偵測方式，以及產生的宣告如何設定；當 `ionic generate modal MODAL_PATH` 產生所要求的帶有 `Modal` 後綴的類別，並在適用時將其加入 `entryComponents`，即可視為完成。

由索引模型根據 Issue 內容生成。