all: add common options schema

[davidsneighbour]

Some plugins have issues adding options to frontmatter. For instance, sitemap as frontmatter will be overridden by the sitemap data of that page. To prevent that from happening, we will start using options as frontmatter particles that can be extended freely. This will be available in page.Params.options and can be prepared with common functions.

Sample:

---
options:
  some_option: true
  some_other_option: ["an" "array" "of" "thing"]
---

[davidsneighbour]

options:
  robots:
    some_option: true
    some_other_option: ["an" "array" "of" "thing"]

is a better way to separate and avoid overriding between modules (robots being the module name or slug).