Added checks to make sure the specified track exists

Added precondition checks to validate input files and paths to return more informative error messages as per #2

node_modules/optimist/test/short.js

@@ -0,0 +1,16 @@
+var optimist = require('../index');
+var test = require('tap').test;
+
+test('-n123', function (t) {
+    t.plan(1);
+    var parse = optimist.parse([ '-n123' ]);
+    t.equal(parse.n, 123);
+});
+
+test('-123', function (t) {
+    t.plan(3);
+    var parse = optimist.parse([ '-123', '456' ]);
+    t.equal(parse['1'], true);
+    t.equal(parse['2'], true);
+    t.equal(parse['3'], 456);
+});