Ability to set headers for Requests

[richard-keller]

When using tus-php in client mode, there is no way to set headers using middleware. It appears that it's only possible to set headers in server mode.

[ankitpokhrel]

Fixed in #90. Guzzle options can be passed via constructor.

$tusClient = new TusClient('http://tus.server', [
    'connect_timeout' => 3.14,
    'allow_redirects' => false,
    'headers' => [
        'User-Agent' => 'testing/1.0',
        'Accept' => 'application/json',
    ],
]);