feat: support exporting a template function

**I'm submitting a feature request**

#### Template (`{Function}`)

**component.html**
```html
<div>${ _.hello }<div>
```

**webpack.config.js**
```js
{
  loader: 'html-loader', options: { template: true }
}
```

**component.js**
```js
import template from './component.html'

const component = template({ hello: 'Hello World!' })
// <div>Hello World!</div>
```
