add new option for use to customize how can useminPrepare task find script and style tags

add new option for use to customize how can useminPrepare task find script and style tags

* name: srcPattern
* Type: 'string'
* Default: /(href|src)=["']([^'"]+)["']/

The RegExp used to find script or link tag, if you are using some server side frameworks to generate this tags, this option maybe useful for you to customize how to find these tags.

For example:
I am using the HtmlHelper from CakePHP to generate script and style tags in my template, the default link finding regex in useminPrepare cannot handle this, because its only effective on hardcoded script and link tags

```php
<!-- build:css /css/src/optimized/bootstrap-fontawesome.css -->
<?php echo $this->Html->css('/vendor/bootstrap/docs/assets/css/bootstrap.css'); ?>
<?php echo $this->Html->css('/vendor/font-awesome/css/font-awesome.min.css'); ?>
<!-- endbuild -->
```