Loading external scripts dynamical is pretty difficult because browsers limit what we can do a lot for security reasons. This leads to the following limitations:
The player has to type in the exact name of the directory for the mod.
We cannot validate if a mod actually exists.
A not existing/broken mod will stop the entire loading process from continuing without us being able to detect it.
Mods have to explicitly declare if they have sub scripts.
Mod scripts have to call the modding API to say they are finished (for every script!)
Mods can only be loaded once SC has finished loading, which means they can't influence the first rendering.
Other notes:
Mods are saved per browser, not per save (similar to hotkeys).
Settings are currently in experimental section of options.