Add Google Tag Manager

[a108167]

I would like to extend this theme to include Google Tag Manager and I'd like to mirror how mkdocs-material did it:
squidfunk/mkdocs-material#2055
Before I submit a PR I wanted to confirm that this repo was still being actively monitored. Thanks.

[dsagal]

To be honest, I've been working with major overrides of the theme that have diverged in a big way, in particular because of issues in getting pages indexed by search engines (the funky redirections may not have been a great idea).

So the version here has gotten no attention for some time. I could push my changes into a branch, if there is interest in a potential v2 of this theme. But also no objections to a Google Tag Manager support to add to this version.

[a108167]

If v2 is inevitable then it would probably make sense to add it in that one. Can you push it into a branch but also let me know your thoughts on timelines, etc? I can then decide if I would like to implement it in v1 or wait.
Thanks!