Add config.apis.gtm configuration for Google Tag Manager.
Add config.apis.gtm configuration for Google Tag Manager.

This is for the Meteor Docs property and one of many commits to address meteor/meteor#8683.

This works in conjunction with https://github.com/meteor/hexo-theme-meteor/commit/cc51c689a3e543e6fd58331efe0e54d7b6262847.

Hansoft/meteor,Hansoft/meteor,Hansoft/meteor,Hansoft/meteor,Hansoft/meteor,Hansoft/meteor,Hansoft/meteor