Improve Comparison::CONTAINS: allow to use custom position for % and _  wildcard characters

This pull prevent forced  wrapping of the value in to `%` 
and allow to use the custom position for `%` and `_`  wildcard characters in case `Comparison::CONTAINS`

Allow to build the `contains` expressions like:
```php
Criteria::expr()->contains('myField', 'some string%');
Criteria::expr()->contains('myField', '%some string');
Criteria::expr()->contains('myField', '10\%');
Criteria::expr()->contains('myField', 's_m_ string');
```