FTS: Include/exclude documents matching conditions from index/results

Taking the description directly from Slack:

Hey folks! :slightly_smiling_face:
Probably as a feature request, but would it be do-able to create indexes which exclude/include documents based on the contents of the document? So say I have an index defined with a separator of :: and a top level field mapping of Customer, I then only want it to index documents where the field alias is missing (alias IS MISSING in N1QL speak)

This is effectively analogous to the where clause of a N1QL query in terms of being able filter results directly