Variable: fix sassdoc for x-inspect()
Moduel: fix typo of variable names
Variable/x-inspect: fix ugly indent of map
Module: add x-module-extend mixin

There are no difference from x-module mixin for now.
Module: @at-root on modifier level

Now selectors are exporesed at root on modifier level
so that type selector isn't exposed at root anymore.

In addition, there are no need to use parent selector (&) in module
definition.
However, x-extend mixin still need parent selector inside it.