Make Fourmolu's use of editor 'tabSize' configurable

[georgefst]

Follow up to #599.

Fourmolu sets indentation based on the editor's setting, when available. Some people (@zweimach) don't like this, so we should have the option to disable this behaviour.

Blocked on #513.

[ndmitchell]

In particular, because fourmulo writes out:

foo = x
  where
    x = 7

I've had the default VS Code tab be detected as 2 for some short documents, which then causes fourmulo to do the unexpected thing.