Provide a way of defining hierarchy of values, first one used

In many cases it would be useful to define a hierarchy of default values, e.g. if field_1 is empty try field_2, if field_2 is empty try field_3, etc.

Note, this is not the same as outputting multiple tags for one meta tag, that use case is handled in #1282636: Support meta tags that allow multiple values (repeated tags).