Perspective config

[texodus]

Adds global config feature to perspective. Config can be defined in 2 ways:

1. In a perspective.config.js, perspective.config.json or in your package.json under the "perspective" key (ala babel, jest, webpack, etc). The webpack plugin and node runtime will both auto-include this file when discovered so the client and server are consistent.
2. As an argument to perspective.worker().

Currently, only type-level default aggregate and filter operator are configurable as tests of the feature itself.

Example perspective.config.js:

module.exports = {
    types: {
        string: {
            filter_operator: ">=",
            aggregate: "distinct count"
        }
    }
};