Add the possibility to filter the results by category

[pdobrescu]

We've had various requests to filter the results by category. However, this may bring the situation where there are not enough posts from a specific category in the number of posts fetched from Google Analytics.
In this case, the proposal is to add in the widget:

• a category selector

If there are not enough results when a certain category is selected:

• add a check-box to auto-populate the list with the overall top posts
• add a check-box to auto-populate the list with the latest posts from that category