feat(example): add example app and blog app with demos

blog_demo.rb

@@ -0,0 +1,27 @@
+$LOAD_PATH.unshift(File.expand_path('lib', __dir__))
+require 'prism'
+require 'net/http'
+require_relative 'examples/blog/app'
+
+server = Prism::Server.new(Blog, port: 0).start
+port = server.port
+puts "blog demo server on port #{port}"
+puts "database file: #{File.join(__dir__, 'examples/blog/db/development.pstore')}"
+puts '-' * 40
+
+Net::HTTP.start('127.0.0.1', port) do |http|
+  create = http.post(
+    '/posts',
+    URI.encode_www_form(title: 'Hello from Prism', body: 'This record was written through the ORM-lite file store.'),
+    'Content-Type' => 'application/x-www-form-urlencoded'
+  )
+  puts "POST /posts -> #{create.code} Location=#{create['location']}"
+
+  listing = http.get('/posts')
+  puts "GET /posts -> #{listing.code}"
+  puts listing.body
+end
+
+server.stop
+puts '-' * 40
+puts 'blog demo done (process about to exit; data stays on disk for the restart check)'