Added the ability to register custom SQL functions

[JoshuaWise]

This is a pending feature.

db.register(function add2(a, b) {return a + b;});
db.prepare("SELECT add2(?, ?)").pluck().get(12, 6); // => 18
db.prepare("SELECT add2(?, ?)").pluck().get("foo", "bar"); // => "foobar"

// throws an Error because the wrong number of arguments were passed
db.prepare("SELECT add2(?)").get(12);
db.prepare("SELECT add2(?, ?, ?)").get(12, 6, 22);

// available options
db.register({
  name: "void", // defaults to function.name
  deterministic: true, // improves performance if applicable (default false)
  varargs: true, // allows any number of arguments to be passed (default false)
  safeIntegers: true, // integer arguments will be Int64s (default false)
}, function () {});
db.prepare("SELECT void(?)").get(55); // => null
db.prepare("SELECT void(?, ?)").get(55, 19); // => null (not an error because varargs = true)