[Proposal] A new blade statement: stack

`@stack` statement provides easy way to `@push` contents into a container from multiple locations.
# Example use-case

 The most beneficial use-case would be to push javascript code into the before `</body>` javascripts. An example:

**layout.blade.php**

    <html>
        <body>
            @include('something')
            @include('someone')
            <script type="text/javascript" src="jquery.js"></script>
            @stack('scripts')
        </body>
    </html>

**something.blade.php:**

    // Only this page needs helloWorld() function so defining this function here makes sense
    <a href="javascript:helloWorld()">Hello!</a>
    @push('scripts')
        <script type="text/javascript">
            function helloWorld() {
                alert('Hello!');
            }
        </script>
    @endpush

**someone.blade.php**

    // this page has a datepicker so jquery.datepicker.js should be included
    <div id="datepicker"></div>
    @push('scripts')
        <script type="text/javascript" src="jquery.datepicker.js">
        <script type="text/javascript">
             $('#datepicker').datepicker();
        </script>
    @endpush

The resulting content for `@stack('scripts')` would be:

    <script type="text/javascript">
        function helloWorld() {
            alert('Hello!');
        }
    </script>
    <script type="text/javascript" src="jquery.datepicker.js">
    <script type="text/javascript">
        $('#datepicker').datepicker();
    </script>