Add Action Tags feature to render highlights as a markdown heading

[chriselderxyz]

Problem:

• The only feature Readwise has that that I need that this plugin doesn't have is rendering the highlighted text as a markdown heading.
• See the Readwise Action Tag feature here - https://docs.readwise.io/reader/docs/faqs/action-tags

This Pull Request:

• Renders highlights as markdown headings when you add a note with the same syntax as readwise (.h1 || .h2 || .h3 || .h4 || .h5 || .h6).
• Puts the ^ref-{{id}} under the heading if it's a heading being rendered (otherwise it looks weird).
• Accounts for the extra line above the ^ref-{{id}} when inserting new highlights above the already rendered ones.

Let me know what you think!