[Proposal] Added @set to easily set variables in templates

Example use case below.

app/views/layout.php:
```html
<html>
<head>
    <title>{{ $title }}</title>
</head>
<body>
    @yield('body')
</body>
</html>
```

app/views/index.php:
```html
@extends('layout')

@set('title', 'Homepage')

@section('body')
    Hello, world.
@stop
```