[4.1] Implement variable name whitelist

[GrahamCampbell]

It is now possible to tell the loader to only allow a predefined set of variables to be loaded. Since whitelisting is a kinda edge case requirement, I'll not be adding special create methods, like I did for choosing between mutable and immutable. Directly newing up Dotenv will be required. This is to keep the interface nice and simple for the majority of users.

A full example is below, assuming the directory where the .env file is stored in $directory and the array of allowed environment variable names (case sensitive) is in $whitelist:

<php

use Dotenv\Dotenv;
use Dotenv\Loader\Loader;
use Dotenv\Repository\RepositoryBuilder;
use Dotenv\Store\StoreBuilder;

$loader = new Loader($whitelist);
$repository = RepositoryBuilder::create()->immutable()->make();
$store = StoreBuilder::create()->withPaths($directory)->make();

$dotenv = new Dotenv($loader, $repository, $store);

$dotenv->load();

---

// cc @ilimic1