Add support for Open Graph Arrays #92
Description
It is possible in Open Graph Protocol to specify more than one value for a single property. It's called OG Array http://ogp.me/#array.
It seems that currently extruct doesn't support arrays in case the uniform option is set to True, because uniform._uopengraph function doesn't handle duplicated keys from the list of properties.
It'd be cool to add that support and return list in case there is more than one value or to just add some separate property with list suffix to be backward compatible.