Add --game command line flag

This allows for the creation/search of directories under a single
"flare" directory. For example, if we launch Flare with `flare
--game=wandercall`: config files will be in
`$CONFIG_DIR/flare/wandercall/`; saves will be in
`$LOCAL_DATA_DIR/flare/wandercall/`; and game data (mods folder) will be
in `$DATA_DIR/flare/wandercall/`.

If the flag is not set, we fall back to the current directory.
Change default data directory in CMakeLists.txt
Rename --game_data flag to --data_path
Don't fall back when --game is omitted
Allow the use of locally installed mods

We can now load mods from the same place we store save games. So if
saves are stored at `$HOME/.local/share/flare/wandercall/`, we can put
mods in `$HOME/.local/share/flare/wandercall/mods/`.
Rename ENGINE_FOLDER to GAME_FOLDER
Merge pull request #574 from dorkster/folders

New directory structure & local mod support