Support creating CLI commands using emuto + oclif

[kantord]

oclif should be an optional dependency. It's already a dependency for emuto-cli, which means it'll be available in the command line every time.

Guide for testing the builtin command in jest: https://martianwabbit.com/2018/05/25/testing-oclif-with-jest.html

The API should be similar to this:

#! emuto -s

command {
  "description": "My command",
  "flags": [
    { "type": "version", "char": "v" }
  ],
  "main": $main
}

where $main = (
  $ => $.stdin.my.filter
)