Add `enable` option for `include` to enable optional including for addtional spec (#1242)

* add new option enable for include of spec

* fix to see the environment variable when parsing include

* add test for include with environment variable

* fix how to parse boolean value

* add spec about enable for include

* add Change Log

* fix the number of PR in changelog

* fix include test to make more clear

* fix test to focus enable option more

* fix english error

* fix to expand variable only one time

* add new test case by setting environment object as NO