Option for meteor settings file

Some applications use a settings file, given via the `--settings` argument to the
`meteor`/`mrt` command or as a JSON string assigned to the `METEOR_SETTINGS`
environmental variable on self-hosted environments.

By providing an argument to --settings it is now possible to provide the
location of the meteor settings file and this is read, stringified and assigned
to the `METEOR_SETTINGS` environmental variable for each app.

Example usage

    laika --settings settings.json

Note: if you're wondering about the white-space changes and new line, my vim does that automatically, I hope you don't mind.