mdline: support --formatter

mdline should support pluggable formatter.

It is similar with [textlint](https://textlint.github.io/).

## Tasks

- [ ] mdline cli support `-f` and `--formatter`
- `mdline -f json -o timelint.json timelint.md`
- Resolve `json` with `@mdline/mdline-formatter-json` or `mdline-formatter-json`
- Similar approach: https://github.com/textlint/textlint/blob/master/packages/textlint/src/engine/textlint-module-resolver.ts

Start at the mdline CLI and compare its formatter-resolution approach with textlint's linked textlint-module-resolver.ts. Add support for -f/--formatter using the documented scoped and unscoped package names, then verify that the JSON example produces timelint.json.

Written by the indexing model from the issue text.