Support formatter option

[yamadayuki]

WHY

We provide the frolint original formatter as default. I supposed that there are users who want to use the ESLint formatter (e.g. stylish, checkstyle, json, etc.).

WHAT

I decided to support formatter option.

The users can specify the formatter in a config. For example in package.json

{
  "frolint": {
    "formatter": "stylish"
  }
}

Or specify as CLI option.

frolint --formatter=checkstyle