Allow setting a custom Guzzle HTTP Client for requests.

[noplanman]

After setting the $telegram object, it is very easy to set a custom Guzzle HTTP Client to allow a custom API URL or simply to modify various parameters (like SSL verification):

Request::setClient(new \GuzzleHttp\Client([
    'base_uri' => 'https://149.154.167.197',
    'proxy'    => 'tcp://123.45.67.12:3128',
    'verify'   => false,
]));

For Windows users, copy this snippet into your hook.php or getUpdatesCLI.php or manager.php file, before the $telegram->handle(); call:

Request::setClient(new \GuzzleHttp\Client([
    'base_uri' => 'https://api.telegram.org',
    'verify'   => false,
]));

Fixes #507
Fixes #511