Allow count to function like sum

[nordfjord]

Many times you get data from a server which has been pre-reduced in some way (e.g. already has a count)
In those cases it would be exceedingly valuable to allow count to simply sum those counts together.

Proposed api:

reductio().count(true); // p.count++;
reductio().count('items') // p.count += v.items;

[nordfjord]

To be clear this is mainly about semantics, I find it weird to call reductio.sum when I'm aggregating counts.

[esjewett]

I think we need to work on a general approach to this. See the list I've started at the end of https://github.com/esjewett/reductio/blob/master/NOTES.md. My thinking right now is a whole separate set of reducers for pre-aggregated data.

[tannerlinsley]

This would be easy with alias-able property names like #35, you could easily just alias the sum aggregation as the count property