Better filtering

[dleppik]

More advanced filtering options, including an easier way to add filtering closures. This may require some real-world usage to explore the solution space.

We'll use mongoDB for inspiration, since that has an increasingly popular JavaScript query syntax.

[dleppik]

Some filter-like things we clearly need:

1. Unions and negation (e.g. everything except "no response")
2. A simple builder for single-field filters, or possibly a filter builder which includes the fields as an argument.

[dleppik]

Geographic data is hard to handle without lat/lon distance filtering. Could add support for https://github.com/darkskyapp/sphere-knn