Fix google tag manager snippet in wrong position (#80).

[rvasquez-flip4new]

Add possibility to render snippets in different positions.

[bumi]

Thanks a lot for your Pull Request. and sorry again that it took soooo long to look at this and get a new version out.
The PR was super valuable but we have changed the logic a bit and introduced a new method on the handler that allows the handler to directly manipulate the html response. This gives the handlers more flexibility for cases like the google tag manager requirements.
#84

The google tag manager is updated for that.
What do you think about that? Hope that helps? Are you still using rack-tracker?