Add ability to render a directory of templates

[jpb]

iidy render path/to/dir will render all YAML files in that directory. All files in the directory will be rendered and outputted as a single, multi-document YAML file.

iidy render file.yaml behaves the same as before.

[jpb]

I think the existing documentation and the updates to iidy render --help should be sufficient:

...
Positionals:
  template  template file to render, `-` for STDIN, can also be a directory of templates                                [required]
...

[jpb]

I've updated the help documentation to clarify that only *.yml and *.yaml files in a directory will be rendered (which was the main thing that was not obvious otherwise).