Add filter aggregations

[johannes-scharlach]

I also had the need for a filter aggregation, but chose a more generic approach than #42.

I'm not sure if it's valid to choose any filter that bodybuilder (or in particular the new FilterBuilder) generates, but in particular the terms filter works quite well like this.

A signature like function filterAggregation(filterType, filterField, filterValue, aggregationName) seemed to simple, since it would essentially reduce the API to one particular filter.

Let me know what you think about this approach!

[danpaz]

Thanks @johannes-scharlach! At a high level this LGTM and I'll take a closer look in the next week or so with more specific feedback.

Also pinging @arielshad in case if you have any thoughts on this approach.

[johannes-scharlach]

@danpaz I'd love to see this in a near-future version of bodybuilder, so that I don't need to maintain my own fork. Would you like me to rebase on top of the latest changes?

[danpaz]

I don't think a rebase is necessary, thanks. Can you add a section to the readme showing how to make a filter aggregation? Can you also add a test to test/index.js as a sort of integration test showing that the feature works as you expected? Maybe that test and the readme can show how you might create this query from the docs: https://www.elastic.co/guide/en/elasticsearch/reference/2.4/search-aggregations-bucket-filter-aggregation.html#search-aggregations-bucket-filter-aggregation

[johannes-scharlach]

The test is slightly different, but the readme looks like the ES documentation :) Let me know if I should change the test also or you expected something different.

[danpaz]

This is great! I think we can move forward with this while we continue to discuss more structural changes separately in #71 (and chances are a more drastic change would have to be a semver major bump anyway).