rewrite in es5 JavaScript to work in older RStudio

[timelyportfolio]

Great work, I saw jExcel and hoped someone would make it into an htmlwidget. I hope you don't mind me barging in without any permission, but I couldn't find any way to contact you.

RStudio viewer did not become modern until very recently, which means many users of older versions of RStudio will not be able to use excelR.

I will submit a pull rewriting JavaScript in ancient form :)

If you agree this is a good thing, then please review that I didn't miss anything. The only real question I had was on style. I could not find this argument listed in the jExcel docs so I could not test.

Thanks!!!!

[Swechhya]

@timelyportfolio I certainly agree with you on this. I did not realize that it was only recently that RStudio viewer started using es6. Also, the package uses jexcel v3 and not v2 and table styling has been documented here. Thank you for the pull request.