hooks: RFC: Hooks that return values

[davidsneighbour]

It would be nice to have a hook return values (or a value). Depending on the hookname (ret-hookname?) the hook could execute the linked layout and return a value or multiple values.

{{ $returnedvalue := partial "func/hooks" "hookname" }}

One main issue is, how to tell func/hooks that a return value is expected and if it's safe enough to just pipe through what the template returns.

[davidsneighbour]

see #20, closing this in favor of filter hooks.