Filter by tag

[LunaFidalgo]

Current behaviour

Now, we can only filter by "document.type", but we have come across the situation where we have different content based on the same custom type. It this case, it would be nice if we can filter directly by tag, instead of doing it by querying by document.type and then filtering by tag in client side.

implementation ideas
In "_prismicQuery" method we should add a new option to filter by tag.