Feature: Allow nested 'keys' option + targeting all object items of an array using index placeholder

[webketje]

Suppose following front-matter:

---
markdown:
  key: "**test**"
faq:
  - q: "# What is Metalsmith?"
     a: An extremely simple, pluggable static site generator.
  - q: "# Why Metalsmith?"
     a: See homepage
---

It is not possible with the keys option currently:

• to target nested variables (=> lodash.get or similar)
• to target a variable amount of array nested items (=> substitute an @index key?)