Add CLI tool

[eemeli]

Closes #479, CC @danielbayley & @antonmedv

Here's the npx yaml -h output, as currently implemented. I'm still contemplating a few additions like a node style normaliser, and the docs will need an update.

yaml: A command-line YAML processor and inspector

Reads stdin and writes output to stdout and errors & warnings to stderr.

Usage:
  yaml          Process a YAML stream, outputting it as YAML
  yaml cst      Parse the CST of a YAML stream
  yaml lex      Parse the lexical tokens of a YAML stream
  yaml valid    Validate a YAML stream, returning 0 on success

Options:
  --help, -h    Show this message.
  --json, -j    Output JSON.

Additional options for bare "yaml" command:
  --doc, -d     Output pretty-printed JS Document objects.
  --single, -1  Require the input to consist of a single YAML document.
  --strict, -s  Stop on errors.
  --visit, -v   Apply a visitor to each document (requires a path to import)
  --yaml 1.1    Set the YAML version. (default: 1.2)

[eemeli]

Added a --visit option. With that, you can define a visitor like:

// strip-styles.mjs
export default {
  Collection(_, node) {
    delete node.flow
  },
  Scalar(_, node) {
    delete node.format
    delete node.type
  }
}

and then apply it:

echo '{"hello":"world"}' | npx yaml --visit ./strip-styles.mjs

for this output:

hello: world