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/minimist/test/default_bool.js

@@ -0,0 +1,35 @@
+var test = require('tape');
+var parse = require('../');
+
+test('boolean default true', function (t) {
+    var argv = parse([], {
+        boolean: 'sometrue',
+        default: { sometrue: true }
+    });
+    t.equal(argv.sometrue, true);
+    t.end();
+});
+
+test('boolean default false', function (t) {
+    var argv = parse([], {
+        boolean: 'somefalse',
+        default: { somefalse: false }
+    });
+    t.equal(argv.somefalse, false);
+    t.end();
+});
+
+test('boolean default to null', function (t) {
+    var argv = parse([], {
+        boolean: 'maybe',
+        default: { maybe: null }
+    });
+    t.equal(argv.maybe, null);
+    var argv = parse(['--maybe'], {
+        boolean: 'maybe',
+        default: { maybe: null }
+    });
+    t.equal(argv.maybe, true);
+    t.end();
+
+})