Added method for adding custom search and replace rules

It is often necessary to add their own rules to search and replace, e.g. replace `&raquo;` with `»`, but it is not possible without creating a wrapper.

This PR add method `add_ent_search_and_replace` for adding custom replacement rules.
This method accept one argument of type array ($rules), which must be looks like this:
```php
array(
    '/&laquo;/i' => '«',
    '/&raquo;/i' => '»'
)
```

Usage example there is in CustomSearchRuleTest.