Support for setting up cross-domain tracking (Google Analytics)

**Is your feature request related to a problem? Please describe.**
Yes, it is, if you want to track your visitors across different sites.
In order to set up properly the cross-domain tracking for Google Analytics, it is mandatory to customize its code, and right now it is not possible to do it in alf.io :-/

**Describe the solution you'd like**
It would be awesome to be able to change the specific Google Analytics code you would like to include in your alf.io site

**Describe alternatives you've considered**
Probably the best (only?) solution is to add a new textbox + checkbox in the backend in order to show the default value that will be rendered by alf.io and if the user need/wants to customize it, just be able to change it.

**Additional context**
[The official documentation by Google for setting up cross-domain tracking](https://support.google.com/analytics/answer/1034342?hl=en)