Select2 default configuration

Add the possibility to set up some default options of Select2 in the
config.yml

I add this possibility cause of the change from 4127f1e. I am using the
element default value for many form in some project, so I've got some
bugs. I think is better to have the possibility to change the default
value.
Array prototype for select2 config
Select2Type initConfig remove, use constructor
Merge pull request #287 from WedgeSama/master

Select2 default configuration