Use dot syntax for Twig filters: select, where, group, order

[allejo]

For Front Matter like the following:

---
category:
  name: eBay
  order: 1
---

Allow filters to use a dot syntax to go into arrays in Front Matter. For example:

{% set categories = collections.posts | order('category.order') %}

Twig filters that should support this:

• select
• group
• order
• where