Add … (ellipsis) as conceal char for rest params / spread operators

[Delapouite]

Hello

ES6 introduce the rest params and spread operators :
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/rest_parameters
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Spread_operator

Visually, they are basically a series of 3 dots : ...

I propose to introduce a new conceal variable to replace this 3 chars into 1, a proper ellipsis char : …

Here on GitHub, with the font used, the difference may not be obvious, but it would be a real gain in vim.

Thanks.

[goatslacker]

Sure