There is currently official modding support, but modding the game is possible to a certain extend.
The developers started an overview on Reddit regarding Data and Tool that are available inclusive some fan made projects.
Interesting for possible mods and external tools:
Built into the game that can run lua commands or directly lua code.
Live view of the game through http port, this is literally the save game data.
Contains all game balancing data that is not decided through the base mod.
Possibility to change game content such as policies, levels and the freeplay.
Save games can be unzipped and some meta data is available via json files.
Possibility to inject mods via dll to unity.