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/loose-envify/cli.js

@@ -0,0 +1,16 @@
+#!/usr/bin/env node
+'use strict';
+
+var looseEnvify = require('./');
+var fs = require('fs');
+
+if (process.argv[2]) {
+  fs.createReadStream(process.argv[2], {encoding: 'utf8'})
+    .pipe(looseEnvify(process.argv[2]))
+    .pipe(process.stdout);
+} else {
+  process.stdin.resume()
+  process.stdin
+    .pipe(looseEnvify(__filename))
+    .pipe(process.stdout);
+}