mdline should support pluggable formatter.

It is similar with textlint.

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