Creation of a Gif Media to be able to extract gif animation based on video sequences (#285)

* Creation of a media to extract video sequences into gif files
* We add a gif method to the class Video to be able to use the Gif Media.
* Parameters where missing in the declaration of the gif function
* One parameter was badly defined in the gif method
* We use the proper media in the method gif
* We add a missing declaration in the Video class
* Update of the README file
* Modification of the README file
* We remove an empty class

src/FFMpeg/Filters/Gif/GifFilterInterface.php

@@ -0,0 +1,20 @@
+<?php
+
+/*
+ * This file is part of PHP-FFmpeg.
+ *
+ * (c) Strime <contact@strime.io>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace FFMpeg\Filters\Gif;
+
+use FFMpeg\Filters\FilterInterface;
+use FFMpeg\Media\Gif;
+
+interface GifFilterInterface extends FilterInterface
+{
+    public function apply(Gif $gif);
+}

src/FFMpeg/Filters/Gif/GifFilters.php

@@ -0,0 +1,24 @@
+<?php
+
+/*
+ * This file is part of PHP-FFmpeg.
+ *
+ * (c) Strime <contact@strime.io>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace FFMpeg\Filters\Gif;
+
+use FFMpeg\Media\Gif;
+
+class GifFilters
+{
+    private $gif;
+
+    public function __construct(Gif $gif)
+    {
+        $this->gif = $gif;
+    }
+}