Fix IE11 in minified version & adopt ES6 in source

[jhildenbiddle]

Fix #4

1. Fixed error related to removing the scriptEl on page load. This bug was a result of how the <script> element reference was stored on the DocsifyCarbon object. The fix is to not store an object reference and instead do a querySelector for the script ID each time.
2. The source file (index.js) has been converted to ES6. Previous, this file was a mash-mash of ES5 and ES6 syntax, so it was already broken in legacy browsers. Adopting ES6 syntax entirely is therefore not a breaking change and provides a better dev experience. The build command listed in the README.md will handle transpiring the ES6 source to ES5 for legacy browsers.
3. There are a few code simplifications I made as well. There are no behavioral changes.

[jhildenbiddle]

BTW, apologies for all of the back-and-forth on these IE11-related updates. Testing the first PR was challenging just because of IE11 access and how https://docsify.js.org/ is configured (it only displays ads on the live site, not in previews). These changes I was able to fully test, so this should be the last update. 🤞

Thanks!