default export should behave differently when it's the only export

Nobody has claimed this yet.

When I don't use esmify and I write
```
module.exports = () => { ... }
```
and I build it like
```
browserify source.js -o dist.js -s "foo"
```

I can just write `foo()` and my function will be executed. However, when I use esmify and I write
```
export default () => {}
```
and build it the same way, `foo` actually represents an object that contains `default` property, so if I want to call it, I should write
```
foo.default()
```

I think that if there is a default export and there are no more exports, that exported value should be set to `window.foo` instead of an object.

Start by tracing how esmify represents an `export default` when browserify builds a bundle with `-s`. Compare that result with the CommonJS `module.exports` case shown in the issue; done means a module containing only a default export exposes the value directly as `window.foo`, while modules with additional exports retain an object shape.

Written by the indexing model from the issue text.