Suggestion: show the markdown rendered result by default and a switcher between markdown model vs html mode

[panhaoyu]

Hello, nice work, thanks for your sharing. When using the plugin, I have a suggestion. Github and Intellij IDEA both provides the preview mode. By default, when opening a markdown file, the preview will be show. If you want to edit, you can switch the mode to the editor. In my opinion, preview as HTML is more important than write, because the notebook is not only to note but more to view. So I think it's a useful function.

[Megre]

The onenote itself is the HTML editor! NoteWidget does provide the preview mode by providing a window on top of onenote window. It's good enough to preview while writing. But it will be better if NoteWidget can remember the scroll scale when ctrl+scroll is pushed within the preview window.