Make sure it also works when included as an npm module

When I uploaded the package to npm, the index.js in the package was
itself placed under node_modules, which made the exclude-node-modules
feature not work at all (so 'use strict' would be applied to all files.

Now we use a substring of __dirname up to the first occurrence of the
string 'node_modules' as the base directory name.

index.js

@@ -2,8 +2,11 @@ var module = require('module')
 var compile = module.prototype._compile
 var path = require('path')
 
+var index = __dirname.indexOf('node_modules')
+var baseDirectory = (index < 1) ? __dirname : __dirname.substr(0, index - 1)
+
 module.prototype._compile = function (content, filename) {
-  var isThirdParty = filename.startsWith(path.join(__dirname, 'node_modules'))
+  var isThirdParty = filename.startsWith(path.join(baseDirectory, 'node_modules'))
   if (!isThirdParty) {
     content = "'use strict';" + content
   }

package.json

@@ -1,6 +1,6 @@
 {
   "name": "auto-strict",
-  "version": "0.0.1",
+  "version": "0.0.2",
   "description": "Add 'use strict'; to all code not in node_modules",
   "main": "index.js",
   "author": "Tor Arvid Lund <[email protected]>",