Merge in highlight removal option

[ryancheu]

There's a pull request on ZhongWen that the owner doesn't want to merge in, but is pretty useful: cschiller/zhongwen#55.

It'd be great if you could merge this change in to ZhongZhong. Due to the canvas rendering change on Google Docs, I've been using Dropbox paper instead so that the extension still works, but Dropbox paper gets very unhappy about the highlighting behavior. I'm using the extension with this change applied locally and it works well.

[PabloRomanH]

Rikaikun manages to do highlighting and still work correctly on Dropbox Paper. Can you look at the way they are doing it? I think that is a much better solution than a new setting that most people are not going to know about.

Actually, Zhongzhong seems to eventually work if you do it 2 or 3 times, it may depend on what you are hovering from/to.