fix: Meteor.settings are not forwarded to the mirror

`METEOR_SETTINGS` only works in packed Meteor apps (I think [this](https://github.com/meteor/meteor/blob/release/0.8.2/tools/run-app.js#L175) is why it doesn't work in dev mode).

So `--settings` must be used. It is not possible to get the original --settings file (I tried only `process.argv`). So the best thing we can do is write the `Meteor.settings` to a file.