Support yaml schema files

We write all out schemas in YAML and currently need an extra step to convert all the files to JSON before calling `jsonschema2md`.

It would be great if an input format could be passed as a flag (or even if setting the extension to `yaml` or `yml` automatically read files as YAML).

Start by locating jsonschema2md's input parsing and command-line entry point, since the issue currently requires converting YAML before calling the tool. Check how input files are loaded and where flags or extensions are handled. Done means YAML schema files can be passed directly, with the existing Markdown output preserved.

Written by the indexing model from the issue text.