Concatenation feature (#287)

* Creation of a feature to concatenate files into a new one.

* Update of the README
* Creation of the tests for the concatenation
* We use an array of videos instead of a path to a text files
* We use the bundle Temporary File System instead of getcwd

src/FFMpeg/Media/Video.php

@@ -204,4 +204,15 @@ class Video extends Audio
     {
         return new Gif($this, $this->driver, $this->ffprobe, $at, $dimension, $duration);
     }
+
+    /**
+     * Concatenates a list of videos into one unique video.
+     *
+     * @param  string $sources
+     * @return Concat
+     */
+    public function concat($sources)
+    {
+        return new Concat($sources, $this->driver, $this->ffprobe);
+    }
 }