The registry's `get_spec()` was already supporting kwargs as a means to
provide information about the source to the spec constructor/factory
function, but the ``SpecHost`` class wasn't capable of accepting any.
This commit rectifies that. The main goal purpose of this is to allow a
bound field (the file attached by ``ImageSpecFileDescriptor``)--and the
attached model instance--to be taken into account during the spec
instance creation.
Related: #156