-Beta branch experimental "display stretch" support for ultra wide screens now has a toggle option under the resolution list at the settings. You can enable it by going to in-game settings from the main menu and clicking select resolution, then make sure you select the option keep aspect ratio so black bars are allowed to appear on the sides when fullscreen.

Note: For this to take effect, make sure you are on the beta branch for Mazgeon on Steam.
You can click on the game settings on your Steam library and click Betas to join.