Add ability to query image by term ID

[benhuson]

At the moment all the functions and filters are geared towards the queried object.

Would be nice to have a way to get images based on term ID.

See:

• Get image by term ID mfields/Taxonomy-Images#27
• Would it be possible to specify the term as an argument instead of relying on get_queried_object? mfields/Taxonomy-Images#20