Module: module-level inheritance with modifiers by `x-extend()` mixin

x-extend() mixin now allows you to automatically extend a module
with the modifiers.
That means you can use x-extend() like `@extend` for a module and
you can extend multiple modules like a mixin in a module definition.
Besides, the output is still clean!

For example, the following module exnteds two modules and their modifiers.

@include x-module('module', true) {
    @include x-extend(x-module('other-module-1'));
    @include x-extend(x-module('other-module-2'));
}