Add intersect & union support in get_(term|tag|category)_link()

[scribu]

​Taxonomy Intersections and Unions support has been around since WP 2.3, but the get_term_link() functions only work with a single term.

Examples:

get_tag_link(array('foo', 'bar'), 'or')  

--> http://example.com/tag/foo,bar

get_tag_link(array('foo', 'bar'), 'and')  

--> http://example.com/tag/foo+bar

[scribu]

The first example should probably be:

get_tag_link(array('foo', 'bar'), 'in')

[filosofo]

This probably deserves its own function. Function overloading makes things confusing and harder to support in my opinion.

Also, the arbitrary combination of taxonomy items seems more like a search query than a link to a particular object's "canonical" URI, which is what the other get_* functions do. In my opinion.

[scribu]

I agree with filosofo that this would be better as a separate function.

But, since there is no permalink support for such queries, one can just use add_query_arg() to construct a URL.