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.

It makes the code so ugly, and since basically every line becomes wider than 80 chars, it would mean having to split all of them over two lines.

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All of this will have to be completely replaced once I get around to working on support for e.g. the Whitespace package, since it'll require lots of trickery and hacking. To support that, and to make it easier to add new overrides, I plan on implementing a sort of _Override Register™_ where every single override will be resolved and invoked.