Compile hello world to javascript

`make hello` will:

* Compile `example/hello.berg` to `tmp/hello.js`
* Run `node tmp/hello.js`

```
# hello.berg
main() = {
  printf("Hello World");
  0
}
```

Not handling of arguments to main yet, and `printf` compiles to `console.log` via ugly hard-coding but there's working JS code generation! And it's doing the exit code stuff and type checks the main return type.