K nearest neighbor for objects

Description

let k = require('./lib/index');

let center = { a: 1, b: 2 },
    neighbor = { a: 1, c: 4 },
    neighbors = [
      {a: 1, c: 5, d: 3},
      {a: 1, c: 4}
    ];

k.neighbor( center, neighbor ); // return 2 for distance from center to neighbor

neighbor = { a: 1, b: 2 };

k.neighbor( center, neighbor ); // return 0 for distance from center to neighbor

k.neighbors( center, neighbors ); // see how it works