Support overriding showInvisibles

Adds support for overriding `showInvisibles`.

Since the targeted function lives on `EditorView`, I needed to refactor the existing code to pass around `EditorView` instead of `Editor`. In an attempt to minimize how ugly this made the code, I leveraged CoffeeScript's _destructuring assignment_ support.

I don't generally stick to the 80 character rule. I prefer a slightly wider 100 character rule (at least, that's my compromise). But if you want the code to stick to 80 characters, I understand. I don't think splitting them over two lines is really that bad, I kind of prefer it that way ... but again, this is just personal preference.

Your override register just sounds like polymorphism to me ... but maybe it'll be clearer when I see it :grinning: