Added sections inheritance of fetched templates while retaining mutation mode

[Abdillah]

Based on #306, implements section as history of mutation. There is an uncertainty in my implementation details: whether to allow multiple mutation with Template::SECTION_MODE_REWRITE mode or just break at the first attempt or some other alternative behavior.

[Abdillah]

I'm still unsure on what to do if multiple rewrites exist, some in the right mutationlog. If we simply merge, the operation on $this->mutationlog will be lost even if APPEND and PREPEND exists.

[Abdillah]

I'm thinking of grouping REWRITE before any mutation. That is consistent with what template system should do in my mind.

layout.php

<?php $this->section('javascript') ?>

page.php

<?php $this->layout('layout') ?>

<?php $this->start('javascript') ?>
  <script src="base-page-script-very-light-weight.js"></script>
<?php $this->end() ?>

<?= $this->fetch('common-component-a') ?>
<?= $this->fetch('heavyweight-component-b') ?>

heavyweight-component-b.php

<?php $this->start('javascript') ?>
  <script src="base-page-script-support-heavy-weight.js"></script>
<?php $this->end() ?>

common-component-a.php

<?php $this->push('javascript') ?>
  <script src="common-component-a.js"></script>
<?php $this->end() ?>

The end result will be,

<script src="base-page-script-support-heavy-weight.js"></script>
<script src="common-component-a.js"></script>