将变量值转成小写字母。 等同于PHP的 `strtolower()`函数。 例子: articleTitle:`Two Convicts Evade Noose, Jury Hung.` 模板: ``` _{$articleTitle}_ _{$articleTitle|lower}_ ``` 输出: ``` Two Convicts Evade Noose, Jury Hung. two convicts evade noose, jury hung. ```