This plugin IJ Marketplace link adds a menu item that allows you to format all project files, or files in current changeset using Scalariform.
After installing the plugin, configure the plugin by adding a .scalariform.conf file to your project root directory. Check the deafault options here.
When editing a Scala file, you will find the reformat options in the Code menu, and there is also a pre-commit hook that reformats changed files before commiting.
Strongly inspired by the now unmaintained plugin by thesamet.