Allow setting the exec editor by env var.

[jwoertink]

Purpose

Fixes #1698

Description

This allows you to set your editor using an env var ENV["EDITOR"] when running exec.

Now you can set it 3 different ways:

# project widde
Lucky::Exec.configure do |settings|
  settings.editor = "mate"
end

# CLI arg
lucky exec -e mate

# ENV var
EDITOR=mate lucky exec

Checklist

• - An issue already exists detailing the issue/or feature request that this PR fixes
• - All specs are formatted with crystal tool format spec src
• - Inline documentation has been added and/or updated
• - Lucky builds on docker with ./script/setup
• - All builds and specs pass on docker with ./script/test