Support a new `relative_include` tag
Trash the subclass
Rename tag to `include_relative`
Define methods for shared vars between include & include_relative
Separate include_relative test out into its own context
Test that includes relative to self are included
Add tests around nasty include attempts
Add documentation on the include_relative tag
Rename method to `resolved_includes_dir`
Sanitize the resolved includes path
Merge pull request #2870 from gjtorikian/support-relative-include
Update history to reflect merge of #2870 [ci skip]