-> and <- aren't replaced with → and ←

If I've somehow missed how to turn it on in the configuration — I fully inspected the generated `~/.stylish-haskell.yaml` to no avail — but it currently seems it's not possible to use stylish-haskell to convert `->` and `<-` into `→` and `←` respectively.

In fact, there are more examples like this, such as `<=` and `>=` not being turned into `≤` and `≥` respectively — I'm aware these won't compile without the `import Prelude.Unicode` import but stylish-haskell could also add that in the imports section (and possibly warn if the `Prelude.Unicode` package isn't available).

All in all, is it possible to customize stylish-haskell to support all or any of the above? Could it even possibly be added to the tool itself so that the above conversions could be enabled with a configuration flag?