Describe scenario with optional multiline arguments

Covers #621
Add support for typehinted arguments in definitions

Fixes #621
Simplify MixedArgumentOrganiser::getParameterNumberWithTypehintingValue()
Remove nedless condition
Make value typehint check more obvious
Fix typehint
Merge pull request #624 from Behat/bugfix/optional-parameters

Introduce support for typehints in parameter organisers