allow to define a pre-existing HTML layout where fields will be inserted

We can define an HTML layout in the FORM tag, it will be preserved by the form directive, and if it contains some tags with a `sf-insert-field` attribute, the corresponding fields will be inserted at this position (the other fields will be generated bellow). Example:
```
<form name="ngform" sf-model="modelData" sf-form="form" sf-schema="schema" sf-decorator="{{decorator}}">
    <p>Main information:</p>
    <ul>
        <li><span sf-insert-field="name"></span></li>
        <li><span sf-insert-field="email"></span></li>
    </ul>
</form>
```

It is a way to allow to build a specific layout manually in a very flexible way.