K2 extension

[johanjanssens]

This PR closes #364 and implements readonly support for K2: articles, categories and fields and attachments through the ext:k2 extension namespace. Following collection models are available:

Collection models

• ext:k2.model.articles
• ext:k2.model.categories
• ext:k2.model.fields
• ext:k2.model.attachments

Example

Setting up an articles collection is very simple. For example to list all the published articles from category with id 1 sorted alphabetically by title you do:

---
collection:
    model: ext:k2.model.articles
    state:
        published: 1
        category: 1 
        sort: title
---

<ul>
<? foreach(collection() as $article): ?>
	<li><?= $article->title; ?></li>
<? endforeach ?>
</ul>

article() template function

Aarticle() template function has been been introduced that allows to get a single article by it's id or alias.

For example to retrieve the article with id = 1

<article>
   <h1><?= article(1)->title ?></h1>
    <p><?= article(1)->text ?></p>
</article> 

For example to retrieve the article with alias = 'my-article'

<article>
   <h1><?= article('my-article')->title ?></h1>
    <p><?= article('my-article")->text ?></p>
</article> 

Defining custom aliases

Using K2 article aliases directly could results into articles not being found it the alias is edited (by accident). To resolve that and to still make it possible to reference an article by a string you can define your own custom static aliases through the configuration

For example define an alias called about-us and link it to article with id = 1

<?php
return array(
    'extensions' =>
    [
	'ext:k2.model.articles'  =>
	[
		'aliases' => [
			'about-us' => 1
		]
	]
   ]
);

You can now do

<article>
   <h1><?= article('about-us')->title ?></h1>
    <p><?= article('about-us')->text ?></p>
</article>