Add commands for generating and comparing JSON schemas

[mdonnalley]

schema:generate

Uses regex to determine the return types of each command and then uses ts-json-schema-generator to generate JSON schemas for those types.

By default the schemas are split into one file per command and placed under the schemas directory. There's a --singlefile flag to put all of the schemas into a single schemas.json file

~/code/plugin-command-snapshot [mdonnalley/schemas] : bin/run schema:generate --help
USAGE
  $ oclif-example schema:generate

OPTIONS
  --filepath=filepath  [default: ./schemas] directory to save the generated schema files; can use "{version}" to insert the current CLI/plugin version
  --singlefile         put generated schema into a single file

schema:compare

Uses just-diff to find differences between the existing schema and the latest schema. Command exists with code 1 if any differences are found

~/code/plugin-command-snapshot [mdonnalley/schemas] : bin/run schema:compare --help
USAGE
  $ oclif-example schema:compare

OPTIONS
  --filepath=filepath  [default: ./schemas] path of the generated snapshot file

[amphro]

Is it useful to have this be the output? What if it was just an array of output files or something like that?