Multiselect

Added multi-select and default options to the eZ Flow select custom attributes

\# Set the default
\# CustomAttributeSelectionDefault_color=green
\# Allow multiple items to be selected
\# CustomAttributeMultipleSelection_color[]=1

Is it possible that the user would want to select no value and we only want to set a default on the first load?  (I'm not sure what the solution would be in that case and/or whether that's a valid use case, but it's something to consider.)