Strip leading whitespace from JavaScript code blocks extracted from HTML

Extracted JavaScript from HTML has incorrect leading whitespace, for example
```html
<!doctype html>
<html>
<head>
  <script type="text/javascript">
    /*jshint indent:2 */
    var foo = 'foo';
  </script>
</head>
<body>
</body>
</html>
```
```
$ jshint --extract=auto index.html
index.html: line 6, col 5, Expected 'var' to have an indentation at 1 instead at 5.
```
because the extraction just strips everything but JavaScript leaving something like
```
|
|    /*jshint indent:2 */
|    var foo = 'foo';
```
I updated the extraction to strip leading whitespace from each line by the amount of whitespace in the first line. Same code will be extracted as
```
|
|/*jshint indent:2 */
|var foo = 'foo';
```