Multi-file templates

[pablito-perez]

As a task template author, expressing a process in a single yaml file limits its overall complexity
Having the possibility of spreading and organizing many actions across several files would allow for more structured code
Design TBD (import modules from a "root" template? unmarshal several yaml files onto the same object? stitch their text content naively? etc...)

[w3st3ry]

I think design must be defined at all because the title seems to be something like subtask plugin.