add a  grep() method to the search class

Small note to add a utility grep() function to our search class. Because almost every time we prepare a regex it is to grep data into it so we end up with code such as:

``` php
$filtered_strings = preg_grep($search->getRegex(), $source_strings);
```

And this would be shorter and more readable IMO:

``` php
$filtered_strings = $search->grep($source_strings);
```
