Feature/nested layouts

A layout file can now call layout(), essentially allowing layout files to be nested.

# Example use:

```php
<!-- layout.tpl -->
<html>
<!-- boiler plate header code -->
<body>
    <div><!-- nav header code --></div>
    <div id="content"><?= $this->content ?></div>
</body>
</html>
```

```php
<!-- two-column-tpl -->
<?php $this->template('layout.tpl'); ?>

<?php $this->start('content'); ?>
<div class="row">
    <div class="col-lg-8"><?= $this->stream ?></div>
    <div class="col-lg-4"><?= $this->sidebar ?></div>
</div>
<?php $this->end() ?>
```

```php
<!--template1.tpl -->
<?php $this->template('two-column.tpl'); ?>

<?php $this->start('stream'); ?>
AMAZING STUFF HERE 
<?php $this->end() ?>

<?php $this->start('stream'); ?>
Kinda boring stuff over here
<?php $this->end() ?>
```

```php
<!--template2.tpl -->
<?php $this->template('layout.tpl'); ?>

<?php $this->start('content'); ?>
<div class="row">
   <div class="col-lg-12">
       I'm just a boring single column page =(.
    </div>
</div>
<?php $this->end(); ?>
```

Sorry if there's a better way to do this, or I'm just being daft.