Add Markdown supports

[ktiays]

This PR adds Markdown supports that renders contents returned from OpenAI server into Telegram's rich text format. Rather than directly rendering the Markdown contents to HTML, we parse them with a pull-mode parser and generate the message entities provided by Telegram. This approach ensures maximum compatibility with Telegram.

Note:
The feature is disabled by default, users may manually enable it in the configuration.

[Lakr233]

lgtm

[unixzii]

There are still a few of tasks need to be done.

• Fix the occasional failure of rendering some messages.
  • Fallback to raw contents when failed to send rendered messages.
• Add "Show Original" button to the rendered messages.
• Update the doc.