Add visitor to initialize value in multi_type_seq

It a visitor to set a value of a certain position of multi_type_seq to
its default value (0 for contin_t, "" for string, etc).
Implement --fs-ignore-exemplar-features user option
Add exemplar feature as input feature for feature selection within moses

Add option to consider the output of the exemplar as a feature
itself. That way we ask feature selection to find features which score
well combined with that exemplar feature.
Add option to subsample the table before feature selection

Not all feature selection algorithms have randomness. In order to add
some randomness to feature selection we offer the option to randomly
subsample the dataset.
Add exemplar feature as initial feature