Settings for MarkDown Extensions in YAML Front Matter

[RoyiAvital]

It might be trivial to other users, but I think it should be written.

If you define the YAML front matter with markdown_extensions:

---
    # Settings overrides
    settings:
        enable_uml: true
        markdown_extensions:
          - markdown.extensions.footnotes
          - markdown.extensions.attr_list
          - markdown.extensions.admonition
          - markdown.extensions.md_in_html
          - pymdownx.highlight:
              pygments_style: github
          - pymdownx.progressbar
          - pymdownx.github
          - pymdownx.caret:
              superscript: false
---

It will override the whole struct. Namely if any extension is defined in the user settings but not there, it won't be enabled.
I thought this functionality is only to enable more extensions but it actually to set the whole settings state.

It makes sense, just needed to be accounted.

[facelessuser]

Yep, it sets the settings at the top key level, I don't believe we do anything clever to merge certain lower levels of the structure.

[RoyiAvital]

Another note about the YAML front matter is remove all indention from the first level settings.
Namely, instead of the above, make sure it looks:

---
# Settings overrides
settings:
	enable_uml: true
	markdown_extensions:
	  - markdown.extensions.footnotes
	  - markdown.extensions.attr_list
	  - markdown.extensions.admonition
	  - markdown.extensions.md_in_html
	  - pymdownx.highlight:
		  pygments_style: github
	  - pymdownx.progressbar
	  - pymdownx.github
	  - pymdownx.caret:
		  superscript: false
---

See #130.