Markdown file charset detection issue

Describe the bug

It seems that the charset detection implementation at L72 of Plugin.cs of the MarkdownViewer plugin would return incorrect file encoding for UTF-8 files without the BOM, and ultimately causing the rendering of markdown files to display corrupted characters for unicode characters, as shown in the image below (The file is the README.md file of this project). However, I do notice that if the majority content of the file is not written in characters in the ASCII range (e.g. in Chinese), the file will be rendered correctly even the encoding is UTF-8 without BOM, so I'm not sure whether it's about the charset detection part or the Encoding.Default part.

Also, related issue #758.

To Reproduce

1. Use QuickLook to preview a markdown file encoded in UTF-8 without the BOM
   

2. If I save the file in UTF-8 with BOM:
   
   The rendering is normal:
   

Expected behavior

Given that "The Unicode Standard neither requires nor recommends the use of the BOM for UTF-8", UTF-8 encoded text files without the BOM should be read correctly.

Desktop Environment

• OS Version: Windows 10 1709
• QuickLook Version: 3.6.11

Start at line 72 of QuickLook.Plugin/QuickLook.Plugin.MarkdownViewer/Plugin.cs and inspect the MarkdownViewer charset detection and Encoding.Default behavior. Reproduce the issue with the project's README.md saved as UTF-8 without a BOM, then verify that Unicode characters render correctly without requiring a BOM.