Add new console commands via ConsoleMethods mixin

[jaredcwhite]

Inspired by Rails' ConsoleMethods technique, this PR adds a Bridgetown::ConsoleMethods module which mixes into IRB's main context. In addition to site, you can now access the collections shorthand, and you can also run reload! which will reload all plugins and reset/reread the site.

Also cool is you can extend ConsoleMethods to add your own commands to the console!

# somewhere in plugins/site_builder.rb:

module ConsoleMethods
  def ruby_rocks
    "MINASWAN!"
  end
end

Bridgetown::ConsoleMethods.include ConsoleMethods

bin/bridgetown c

irb> ruby_rocks
=> "MINASWAN!"

[render]

Your Render PR Server URL is https://bridgetown-api-pr-439.onrender.com.

Follow its progress at https://dashboard.render.com/static/srv-c63ccskobjd1u6vtf280.

[render]

Your Render PR Server URL is https://bridgetown-beta-pr-439.onrender.com.

Follow its progress at https://dashboard.render.com/static/srv-c63cct4objd1u6vtf2bg.