views: support multiple values for metadata

For the moment only the last value is used in views. This patch add support for more values. It adds two options to the 'metadata blocks' fields in views. This allows the user to configure what should happen with the multiple values. Either the multiple values are kept as an array in order to let the theming function deal with it. Or the values are joinned into a single string using the specified join string.
I'm not entirely sure if similar logic should be added to other views handlers as well.