Add slim as dev dependency and suggested package
Refactor Slim middleware

* The entire public API should be documented.
* The middleware itself is responsible for handling configuration.
* Using `$app->add($middleware)` makes it more obvious what's going on
  behind the scenes and allows for a more consistent experience within
  Slim applications.
* The `Honeybadger::$config->user_information` is formatted with the error ID
  and replaces a `<!-- HONEYBADGER ERROR -->` placeholder if an exception is
  handled and placeholder is within the response body.
Fix typo in Slim logger
Allow access to wrapper logger and threshold
Clean up and add specs to Slim