Add barebones example game

Author: Artur Ostręga

example/Gemfile

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+source 'https://rubygems.org'
+
+gem 'yeah', github: 'yeahrb/yeah'
+#gem 'yeah', path: '../'
+gem 'yeah-web', github: 'yeahrb/yeah-web'
+gem 'opal', github: 'opal/opal'

example/README.md

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+# Example Game
+
+A barebones example game included with Yeah.
+
+## Usage
+
+Install the game's dependencies.
+
+`$ bundle install`
+
+Start a web server for the game.
+
+`$ bundle exec yeah server`
+
+Finally, visit http://localhost:1234 to run the game in your web browser.
+
+Note: graphical output is in the works.

example/src/game.rb

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+class ExampleGame < Yeah::Game
+  def initialize
+    puts "Hello!"
+  end
+end