Allow the configuration of error messages

[bruno-brant]

Assertion messages are composed automatically from the name of the field. They aren't localized, for instance. Allowing users to configure the message would be nice to support other languages and/or custom messages.

[bruno-brant]

I have a few ideas concerning how to do it:

Directly on the API

let assert = require("assert-plus");
assert.object(foo, "foo", "%s can't be null");
// or
assert.object(foo, "foo", "%s deve ser preenchido"); // in pt-BR

Globally

let assert = require("assert-plus");
assert.messageTemplate = "%s can't be null";

Locale sensitive

let assert = require("assert-plus");
assert.locale = "pt-BR";
assert.messageTemplate = { "pt-BR": "%s é requerido" };

If someone can provide me with some feedback on the methods, maybe I can slap together a PR during this week.