Breaks IE11 (which is supported by docsify v4)

[jhildenbiddle]

The docsify-dark-mode plugin is using ES2015 features which are not supported by IE11.

Since IE11 is officially support by docsify, docsify plugins should be authored using only ES5 or transpiled from ES6+ to ES5 using a tool like Babel.

Thanks!

[anikethsaha]

Yeah, I did forget that. I will fix that in next release.
Also, It is removed from the docs site. SO should be fixed in the next release of docsify. docsifyjs/docsify#1207.