Google analytics tracking code extracted to seperate file

I have extracted the analytics script generation to be manually included in `_includes/_google-analytics.html`

The reasoning behind this is google analytics has [different variations](https://github.com/kippt/jekyll-incorporated/issues/8) of scripts, which makes it complex to give a standardized solution based on property id.

So, I've added a file, where users can place their flavor of analytics code.