Support mutually exclusive options

[gustavothecoder]

Resolve #128

This PR adds support to mutually exclusive options (i.e. options that cannot be used together). The implemented API is exemplified bellow. I'm not totally satisfied with the current API, so any suggestions will be appreciated.

require "dry/cli"

class Greetings < Dry::CLI::Command
  mutually_exclusive_options [
    [:english, {desc: "Chooses English", type: :flag}],
    [:spanish, {desc: "Chooses Spanish", type: :flag}],
    [:portuguese, {desc: "Chooses Portuguese", type: :flag}]
  ] 

  def call(options)
    if options.key?(:english)
      puts "Good morning"
    elsif options.key?(:spanish)
      puts "Buenos días"
    elsif options.key?(:portuguese)
      puts "Bom dia"
    end
  end
end

# $ foo greetings --english
# Good morning

# $ foo greetings --english --spanish
# ERROR: "foo greetings" was called with arguments "--english --spanish"