FEAT: Add HTML compose mode toggle

[andrinoff]

Is your feature request related to a problem?
Markdown composition only. No rich HTML compose mode.

Describe the solution you'd like
Toggle between plaintext/markdown and HTML compose. Rich text formatting.

Describe alternatives you've considered
Markdown only (current, simpler).
External HTML editor.

Additional context
Many users prefer WYSIWYG. Could use terminal formatting or external preview.