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/dash.js

@@ -0,0 +1,31 @@
+var optimist = require('../index');
+var test = require('tap').test;
+
+test('-', function (t) {
+    t.plan(5);
+    t.deepEqual(
+        fix(optimist.parse([ '-n', '-' ])),
+        { n: '-', _: [] }
+    );
+    t.deepEqual(
+        fix(optimist.parse([ '-' ])),
+        { _: [ '-' ] }
+    );
+    t.deepEqual(
+        fix(optimist.parse([ '-f-' ])),
+        { f: '-', _: [] }
+    );
+    t.deepEqual(
+        fix(optimist([ '-b', '-' ]).boolean('b').argv),
+        { b: true, _: [ '-' ] }
+    );
+    t.deepEqual(
+        fix(optimist([ '-s', '-' ]).string('s').argv),
+        { s: '-', _: [] }
+    );
+});
+
+function fix (obj) {
+    delete obj.$0;
+    return obj;
+}