refmt breaks things

Hi @jaredly

I know this is alpha, but i've already played with it a bit :D Thanks for working on this, it is very big thing!

I've noticed that if I call refmt on the file with macros it breaks ppx.

To reproduce:

refmt this file https://github.com/jaredly/reason-macros/blob/master/tests/KitchenSink.re

I was using refmt from bs-platform 5.0.6

```
Fatal error: exception Location.Error(_)
File "~/src/Shared__Macros.re", line 1:
Error: Error while running external preprocessor
Command line: ~/ppx.sh '/var/folders/44/471dqp2577x_j7s71d2q01f80000gn/T/camlppx249433' '/var/folders/44/471dqp2577x_j7s71d2q01f80000gn/T/camlppx2f2e41'

```

Start by running refmt on tests/KitchenSink.re with bs-platform 5.0.6 and inspect the reported ppx.sh failure in Shared__Macros.re. Trace how refmt handles the macros and reproduce the Location.Error; done means the file formats without the external-preprocessor failure.

Written by the indexing model from the issue text.