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

README.md

@@ -441,6 +441,18 @@ $video->save($format, 'video.avi');
 
 The callback provided for the event can be any callable.
 
+##### Add additional parameters
+
+You can add additional parameters to your encoding requests based on your video format.
+
+The argument of the setAdditionalParameters method is an array.
+
+```php
+$format = new Format\Video\X264();
+$format->setAdditionalParameters(array('foo', 'bar'));
+$video->save($format, 'video.avi');
+```
+
 ##### Create your own format
 
 The easiest way to create a format is to extend the abstract