feat: add functions support native promise

[eerFun]

I wrote a native ES6+ promise wrapper around all functions in 'promise.js'. And, then import them in main file for exporting from the module.

• [x] docs(README): add usage and examples docs, try to follow your rules
• [x] docs(README): set language for all code sections for syntax highlighting

Issue: #4 functions support native ES6+ promise

[harshankur]

Hey, I am sorry for my late reply. Thank you for your contribution. I have gone through your code. I like your idea of converting callbacks to promise versions. I, however, find that your code produces a cyclical import problem with both files importing from each other.

Try putting all your code from promise.js into officeparser.js and remove all the importing code and it should solve the issue.

If you want to be added as a contributor, change as above and send me another pull request. Or if you just want the feature added into the library, let me know and I will add it myself and update the package.

Let me know.