4.x - Route InvocationStrategy Setter

[l0gicgate]

This is pull 3 out of 7 to complete the goals set in #2604

This PR adds the capability to set a Route's invocation strategy per route.

Closes #2597

Example usage:

use Slim\App;
use Slim\Handlers\Strategies\RequestResponse;

$app = new App(...);

$invocationStrategy = new RequestResponse();
$app
    ->get('/', function () {...})
    ->setInvocationStrategy(...);

[coveralls]

Coverage increased (+0.004%) to 98.516% when pulling 2cf746c on l0gicgate:4.x-RouteInvocationStrategySetter into 8e07064 on slimphp:4.x.