Fix Issues about the Markdown Plugin

[hyrious]

Is your feature request related to a problem? Please describe. All issues related to Markdown. I'm not able to debug QuickLook in my visual studio. So I'll put some suggestions here.

Describe the solution you'd like

• [ ] The default charset is not UTF8. It may be fixed by changing https://github.com/QL-Win/QuickLook/blob/df66c425e30adbc349c8ba3d681edc780b41a65d/QuickLook.Plugin/QuickLook.Plugin.MarkdownViewer/Plugin.cs#L72

  - var encoding = CharsetDetector.DetectFromBytes(bytes).Detected?.Encoding ?? Encoding.Default;
  + var encoding = CharsetDetector.DetectFromBytes(bytes).Detected?.Encoding ?? Encoding.UTF8;

• [ ] (#752, #728, #693, #362) Switch to KaTeX for better performance.
• [ ] (#742) Dark theme. You can use my fork of github-markdown-css. Also, #836 will be resolved by this css file.
• [ ] (#508, #82) Not able to read local image file. The best solution I can imagine is starting a local http server in the markdown file's folder, which is possible with <Capability Name="privateNetworkClientServer" />. A quick implementation in Node.js (with hot reload support) can be find here.

[ghost]

I cannot find Plugin.cs in QuickLook/QuickLook.Plugin/QuickLook.Plugin.MarkdownViewer, only QuickLook.Plugin.MarkdownViewer.dll & UtfUnknown.dll

[hyrious]

I cannot find Plugin.cs in QuickLook/QuickLook.Plugin/QuickLook.Plugin.MarkdownViewer, only QuickLook.Plugin.MarkdownViewer.dll & UtfUnknown.dll

...Source code...

[Alaknar]

I'd LOVE for Markdown files to be displayed in dark mode.