Support searching with facets.

[dojutsu-user]

Search facets will be useful in many ways.
They can be used to target the search inside a subproject.
For example: Celery has a subproject Kombu, so we can search in Celery docs, we can search inside Kombu docs, like: subproject: kombu serializers

[stsewd]

Other type of facets/filters we could support

• :dir:dev/ to search only on the dev/ directory
• :project:subproject-alias to search on the main project or a subproject

[stsewd]

We already support this syntax, the only thing missing is searching by sub-directories. I'll create a different issue for that.