New directory structure & local mod support

This PR has a few changes:

* Added a `--game` flag select a set of mods. See this: https://github.com/clintbellanger/flare-engine/issues/568#issuecomment-16060334
* Following the style of the new mod layout, configs/user data are now stored in subfolders by game (ex: `~/.config/flare/wandercall/`)
* Added support for locally installed mods. A `mods/` folder in the same place where save files are located. Locally installed mods take priority over system-wide mods
* Renamed the `--game_data` switch to `--path_data` for clarity