optionally hit worker even if some sources didn't hold data for the tile

Nobody has claimed this yet.

Per [this code](https://github.com/mapbox/tile-reduce/blob/5aaf09f9b196f4d5fe0fda9f76884e75b7c37db8/src/worker.js#L42-L45):

``` js
for (var i = 0; i < results.length; i++) {
data[sources[i].name] = results[i];
if (!results[i]) return process.send({reduce: true});
}
```

the worker bails out and returns a reduce event if any source doesn't have data for the requested tile. This is usually great, but in some cases where you want to compare disparate data sources and are relying on reduce events to send back information about how much data each source does or doesn't exist in a tile, you end up losing information.

For example, if I want to find the length of roads in San Francisco that are matched by GPS datapoints. I would like to keep a tally of the total length of road in the bbox, as well as how much is matchable by GPS points. Right now, if there is no GPS data in the tile, we bail out, so I'm missing some of the total length information.

To maintain compatibility and provide optimization for the usual cases where you _want_ this bail-out behavior, I'm proposing we add a tile-reduce option for this, maybe `requireAllSources: false` (defaulted true).

cc @morganherlocker @aaronlidman @mourner

Start with src/worker.js at the loop in lines 42-45, then trace how tile-reduce options reach the worker and how reduce events are emitted. Add an option that preserves the current default bail-out behavior while allowing processing when a source has no data, and verify that reduce events retain the available source information.

Written by the indexing model from the issue text.