Methods getAdditionalParams & setAdditionalParams which allow the user to pass additional parameters to the encoding request (#284)

* Modification of the format Video to add additional parameters based on user's desire
* Update of the README
* Working version of this feature. Still needs tests
* Fixing the tests of FFMPeg\Media\Video
* Setting up tests for the additionalParams feature
* Correction des tests
* Modifying tests. They work locally but not on Travis.
* Still trying to understand why Travis is throwing errors when PHPUnit is not.
* Add the additional params at the end of the command
* Fixed the tests and the way we add the parameters
* We remove log files

src/FFMpeg/Format/VideoInterface.php

@@ -54,4 +54,11 @@ interface VideoInterface extends AudioInterface
      * @return array
      */
     public function getAvailableVideoCodecs();
+
+    /**
+     * Returns the list of available video codecs for this format.
+     *
+     * @return array
+     */
+    public function getAdditionalParameters();
 }