名称： article_categorys
功能： 读取所有文章分类
返回值： 多维数组，若存在多级分类则会格式化为树形结构
数据格式示例：
Array
(
[1] => Array
(
[cat_id] => 1
[category] => doc
[parent_id] => 0
[unique_id] => doc
[cat_name] => 开发文档
[title] => ECMS开发文档
[keywords] => ecms开发文档,ecms参考文档,ecms使用手册
[description] => ecms开发文档
[content] =>
[sort] => 0
[category_url] => /doc/p1-articles.html
[children] => Array
(
[3] => Array
(
[cat_id] => 3
[category] => doc
[parent_id] => 1
[unique_id] => 快速上手
[cat_name] => 快速上手
[title] => 快速上手
[keywords] =>
[description] =>
[content] =>
[sort] => 0
[category_url] => /快速上手/p1-articles.html
)
[4] => Array
(
[cat_id] => 4
[category] => doc
[parent_id] => 1
[unique_id] => 模板手册
[cat_name] => 模板手册
[title] => 模板手册
[keywords] =>
[description] =>
[content] =>
[sort] => 0
[category_url] => /模板手册/p1-articles.html
)
)
)
[2] => Array
(
[cat_id] => 2
[category] =>
[parent_id] => 0
[unique_id] => blog
[cat_name] => 博客
[title] => 博客
[keywords] => ...
[description] => ...
[content] =>
[sort] => 0
[category_url] => /blog/p1-articles.html
)
)