Allow pipelines to require at least some external context

Initially, i'd been thinking that pipelines would only be able to operate on global contexts (current user, etc.). however, Earl made the point that we do need to be able to let pipelines account for things like node access, as in the case of Panelizer, it usually makes the most sense to tie access to the IPE to the same node access 'edit' perm as is used for editing the node itself.

this means introducing the same kind of context handshaking method onto the pipelines system as we currently use with minipanels.