Compatibility with esm

The line: https://github.com/mscdex/mmmagic/blob/master/lib/index.js#L2 produces:

```
ReferenceError: __dirname is not defined
```

[There is a discussion for webpack](https://github.com/webpack/webpack/issues/14072), but it would be great, if esm could be supported by this great library.

BTW, if I understand correctly, it uses a very old version of libmagic. Maybe it could be updated.