add profile support for selenium2 when using firefox

Logic is already available in MinkSelenium2Bundle but could not be set via config.
With setting profile you can:
* use a mobile user agent
* use preconfigured firefox extensions
* etc.

see also: https://github.com/Behat/MinkSelenium2Driver/blob/master/src/Behat/Mink/Driver/Selenium2Driver.php
line 99

PS: I'm not serious about line 118
defaultNull()->
Merge remote-tracking branch 'origin/patch-1'