Added custom filters for Audio, in the same way as custom filters are available for Video

tests/Unit/Filters/Audio/CustomFilterTest.php

@@ -0,0 +1,20 @@
+<?php
+
+namespace Tests\FFMpeg\Unit\Filters\Audio;
+
+use FFMpeg\Filters\Audio\CustomFilter;
+use FFMpeg\Filters\Audio\FrameRateFilter;
+use Tests\FFMpeg\Unit\TestCase;
+use FFMpeg\Coordinate\FrameRate;
+
+class CustomFilterTest extends TestCase
+{
+    public function testApplyCustomFilter()
+    {
+        $audio = $this->getAudioMock();
+        $format = $this->getMock('FFMpeg\Format\AudioInterface');
+
+        $filter = new CustomFilter('whatever i put would end up as a filter');
+        $this->assertEquals(array('-af', 'whatever i put would end up as a filter'), $filter->apply($audio, $format));
+    }
+}