Programmatic execution prints to stdout

protobuf.js version: 6.7.0 (current `HEAD` on master)

When running the sample from the README:
```js
var pbjs = require("protobufjs/cli/pbjs");

pbjs.main([ "--target", "json-module", "path/to/myproto.proto" ], function(err, output) {
    if (err)
        throw err;
    // do something with output
});
```
then `output` is `undefined` and the result gets printed on `stdout` instead.