FIXES#62 - Add StandardRB

[deepakmahakale]

• Add standard as a development dependency to the project.
• Fix lint errors using

  bundle exec standardrb --fix

• Add a GitHub action to ensure PRs are correctly formatted

[schneems]

Getting a failure in bundle install

bundle install
  /opt/hostedtoolcache/Ruby/2.7.6/x64/bin/bundle config --local path /home/runner/work/maildown/maildown/vendor/bundle
  /opt/hostedtoolcache/Ruby/2.7.6/x64/bin/bundle config --local deployment true
  Cache key: setup-ruby-bundler-cache-v4-ubuntu-20.04-ruby-2.7.6-gemfiles/rails_6_0.gemfile.lock-915780d386ff221f4710409e7055ee737bc04d66b384abf78a9b31cf767e74d5
  /opt/hostedtoolcache/Ruby/2.7.6/x64/bin/bundle install --jobs 4
  You are trying to install in deployment mode after changing
  your Gemfile. Run `bundle install` elsewhere and add the
  updated gemfiles/rails_6_0.gemfile.lock to version control.
  
  If this is a development machine, remove the
  /home/runner/work/maildown/maildown/gemfiles/rails_6_0.gemfile freeze
  by running `bundle config unset deployment`.
  
  The dependencies in your gemfile changed
  
  You have added to the Gemfile:
  * standard
  Took   0.71 seconds
Error: Error: The process '/opt/hostedtoolcache/Ruby/2.7.6/x64/bin/bundle' failed with exit code 16
    at ExecState._setResult (/home/runner/work/_actions/ruby/setup-ruby/v1/dist/index.js:6023:25)
    at ExecState.CheckComplete (/home/runner/work/_actions/ruby/setup-ruby/v1/dist/index.js:6006:18)
    at ChildProcess.<anonymous> (/home/runner/work/_actions/ruby/setup-ruby/v1/dist/index.js:5900:27)
    at ChildProcess.emit (node:events:390:28)
    at maybeClose (node:internal/child_process:1064:16)
    at Process.ChildProcess._handle.onexit (node:internal/child_process:301:5)

I'm not sure if this is github's problem or ours.