[Feature] add button to export html

[MagnoAlberto]

Feature description

Add a button for exporting to HTML, similar to exporting PDF, with the option of saving the file with the name of the Thread title.

Motivation

I believe that the use of HTML is much more practical in many situations than in PDF.

[lencx]

I am implementing an export markdown feature, which may make more sense than html, and I can provide a markdown editing preview interface specifically for it.

[Manamama]

See my quick tip for this here - maybe too obvious.

[lencx]

Please try v0.9.2. Exporting markdown is currently supported, and the next step is to implement editing markdown files.