Publish v1.3.4 release

A excellent markdown preview extension for Visual Studio Code, Version 1.3.4.
https://marketplace.visualstudio.com/items?itemName=jhuix.markdown-preview-showdown

1. Update @jhuix/showdowns v0.5.3.

2. Add delimiters setting of katex math features.

3. Add flavor setting of preview html features.

4. Optimized preview html page.