Feat: Ability to post by using markdown type

[tassoman]

Writing posts in markdown style is extremely useful and fully supported by instances different from mastodon.

• Gotosocial: https://docs.gotosocial.org/en/latest/user_guide/posts/#input-types
• Pleroma / Rebased: https://docs-develop.pleroma.social/frontend/user_guide/posting_reading_basic_functions/#rich-text

User stories

• as user I'd like to write new posts by formatted markdown syntax
• as user I'd like to reply posts inline and formatting them by markdown
• as user I'd like to push a button to enable markdown before sending post
• as user I'd like to see a preview of my enriched post, before posting

Final user story maybe: by clicking post, you get a modal window displaying post's rendering having two buttons: "back" and "post".
Default action should be "post".

Inside configuration pane, you should have a checkbox "avoid markdown previews before posting" if checked, does simply post without preview.