added configuration settings for open/close content/escaped content tags

Added extra configuration parameters to allow for custom blade syntax tags

Updated config.php
```
<?php return array(

    'enabled' => true,
    'blade' => [
        'contentTagOpen'         => '{{',
        'contentTagClose'        => '}}',
        'escapedContentTagOpen'  => '{{{',
        'escapedContentTagClose' => '}}}'
    ]

);
```

Addresses the following issues:
fitztrev/laravel-html-minify#9 and fitztrev/laravel-html-minify#15