add variables (+metatag)

I was thinking about adding more variables like:

Error pages - make it also domain variable - with option to do so (so a domain2.com can have error pages on domain2.com and not on domain1.com) - EDIT fixed here https://www.drupal.org/node/2503781 (without an option)

Support for METATAG module - for example:

Meta Description
Abstract
Key Word
...

make them domain variables. Nice start here - https://github.com/kaido24/mgl/blob/master/README.txt

For now Maintenance mode message (admin/config/development/maintenance), Site name [remember to toggling off "Override site name with name of domain." - in admin/structure/domain/settings] (admin/config/system/site-information), Slogan, E-mail address, Front page (admin/config/system/site-information) are working fine.

Related:
https://www.drupal.org/node/1282620
https://www.drupal.org/node/1856898#comment-7054610

Something to think of (not related):
https://www.drupal.org/node/1993588
https://www.drupal.org/node/2308283