Subscriptions: Option to filter what is included in subscription count

After #800, I can understand the suggestion to make the combined count optional. I don't think it is something (at this time at least) to make a UI feature, but I think a filter is worth consideration.

Idea:
1. Add filter to jetpack to default to combined or subscriptions only. I do not want to make it more complicated than that because of the level of code complication that could develop on the API side.
2. Add an argument to the API to accept the type of request, default to combined for BC, and return as such.

No files, tests, or entry points are named. Start by tracing how Jetpack currently computes the combined subscription count and how the API handles that request; done means supporting a filter for combined versus subscriptions-only results while preserving the combined default for backward compatibility.

Written by the indexing model from the issue text.