Better executable code syntax in MD

 

**Problem**

This is with reference to `executeScript` property. Currently, docsify checking an executable script tag using [this](https://github.com/docsifyjs/docsify/blob/develop/src/core/render/index.js#L15-L17)
The markdown input for these doesn't look good [ref](https://github.com/docsifyjs/docsify/blob/develop/docs/themes.md#click-to-preview)

**solution**

if we are moving with pluggable markdown parser then create a plugin ( or use if exists ) to add custom syntax to uniquely identify some pre-coded syntax which can be replaced in the renderer.