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/example/default_singles.js

@@ -0,0 +1,7 @@
+#!/usr/bin/env node
+var argv = require('optimist')
+    .default('x', 10)
+    .default('y', 10)
+    .argv
+;
+console.log(argv.x + argv.y);