Adds external audio parsing and other fixes

Trims subtitle tiles before checking if it's full
Generalizes some stream parsing
Fixes an exception when a stream tag is set, but has no value
Looks for both subtitle and audio streams adjacent to the main file

src/main/java/net/knarcraft/ffmpegconverter/property/StreamType.java

@@ -0,0 +1,28 @@
+package net.knarcraft.ffmpegconverter.property;
+
+/**
+ * A representation of different stream types
+ */
+public enum StreamType {
+
+    /**
+     * A video stream
+     */
+    VIDEO,
+
+    /**
+     * An audio stream
+     */
+    AUDIO,
+
+    /**
+     * A subtitle stream
+     */
+    SUBTITLE,
+
+    /**
+     * None of the above
+     */
+    OTHER
+
+}