feat: allow custom templates in overlay components

**Ionic version:**
"@ionic/angular": "^4.2.0",

# Feature Request
I use an html template with tag ion-select-option and It always render to plain text not html.
```html



{{acc.public_title}}


{{acc.currency}}



```

**Describe Preferred Solution**
Render html not text in ion-select-option

Start by reproducing the provided Angular example with ion-select and ion-select-option, then inspect the select and overlay component entry points involved in rendering option content. Done means custom option templates render as HTML rather than plain text, while existing select behavior remains intact.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.