Add `meltano create <extractor|loader> <name>` CLI command to create a new tap/target from a template
To start, we could use cookiecutter with https://github.com/singer-io/singer-tap-template and https://github.com/singer-io/singer-target-template, until we have a better option (see meltano#2401 (closed) and meltano#2402 (closed)).