Modules and functions have return values

Modules can now use return values with the exit_module function.
Some return values are predefined, use them by passing special string
to the function.

Most of the functions also switched from a echo "true" to return 0
scheme, which means that every test involving theses functions was
rewritten.

No more need for the reset_vars function, as modules are run in
subshells, thus providing a new scope level.

The case when no modules is specified is better handled, a LÖVE file is
created by default.

scripts/windows.sh

@@ -1,7 +1,5 @@
 # Windows
-if [[ $(execute_module "windows") == false ]]; then
-    return
-fi
+execute_module "windows"
 
 init_module "Windows"