Refactor frontmatter - Allow 3rd party plugins to filter frontmatter

[webdados]

Context

• See Allow 3rd party plugins to filter frontmatter #24

Summary

This PR can be summarized in the following changelog entry:

• Refactors the frontmatter generator to allow 3rd party plugins to filter it

Test instructions

Install WooCommerce and https://github.com/webdados/markdown-alternate-woocommerce and check the MD version of a product

Fixes #24

[webdados]

There was a conflict with the new format_taxonomy_terms method that was implemented after I wrote this code, which I resolved by maintaining the more abstract items array entry, which is usable for taxonomies or any other yaml entry that needs "children" (like name and URL in taxonomies).