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/node-modules-regexp/readme.md

@@ -0,0 +1,32 @@
+# node-modules-regexp [![Build Status](https://travis-ci.org/jamestalmage/node-modules-regexp.svg?branch=master)](https://travis-ci.org/jamestalmage/node-modules-regexp)
+
+> A regular expression for file paths that contain a `node_modules` folder.
+
+
+## Install
+
+```
+$ npm install --save node-modules-regexp
+```
+
+
+## Usage
+
+```js
+const nodeModules = require('node-modules-regexp');
+
+nodeModules.test('/foo/node_modules/bar.js');
+//=> true
+
+nodeModules.test('/foo/bar.js');
+//=> false
+```
+
+
+## API
+
+The returned value is a regular expression, [soooo....](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/RegExp).
+
+## License
+
+MIT © [James Talmage](http://github.com/jamestalmage)