[ENH] Add ability to enforce specific filter on Google Analytics query
Fix for #3505

This PR allows to directly write filters to ga.py `read_ga`  function. 

Currently only **OR** filters are supported. with this PR we can support directly writing the filters as a string, allowing for example **AND** filtering.

**Example here**

![example](http://i.imgur.com/28FojWh.png)
