scaffolding like tooling for writing charms

expand "charm create" to have a subcommand, or some equivalent mechanism, to stub out the files for hooks when adding a relation. Something like "charm add-relation [provides|requires|peers] relation-name interface-name, where that would create the relevant hooks and update the YAML with the relation name.

Alternately, charm-stub-hooks which reads the YAML file and stubs out the hooks needed for the relations defined in the YAML file.