[MODEL] Added an example with Edge NGram mapping for auto-completion

This example uses an [Edge Ngram](https://www.elastic.co/guide/en/elasticsearch/reference/current/analysis-edgengram-tokenizer.html)
mapping to demonstrate how to create a custom "settings" for a model,
and how to use the NGram feature for autocomplete.

NOTE: The "Completion Suggester" should be used in all cases where it's
      applicable; see https://www.elastic.co/guide/en/elasticsearch/reference/current/search-suggesters-completion.html.

Closes #582

elasticsearch-model/examples/activerecord_mapping_edge_ngram.rb

@@ -0,0 +1,101 @@
+require 'ansi'
+require 'sqlite3'
+require 'active_record'
+require 'elasticsearch/model'
+
+ActiveRecord::Base.logger = ActiveSupport::Logger.new(STDOUT)
+ActiveRecord::Base.establish_connection( adapter: 'sqlite3', database: ":memory:" )
+
+ActiveRecord::Schema.define(version: 1) do
+  create_table :articles do |t|
+    t.string :title
+    t.date   :published_at
+    t.timestamps
+  end
+end
+
+class Article < ActiveRecord::Base
+  include Elasticsearch::Model
+  include Elasticsearch::Model::Callbacks
+
+  article_es_settings = {
+    index: {
+      analysis: {
+        filter: {
+          autocomplete_filter: {
+            type: "edge_ngram",
+            min_gram: 1,
+            max_gram: 20
+          }
+        },
+        analyzer:{
+          autocomplete: {
+            type: "custom",
+            tokenizer: "standard",
+            filter: ["lowercase", "autocomplete_filter"]
+          }
+        }
+      }
+    }
+  }
+
+  settings article_es_settings do
+    mapping do
+      indexes :title
+      indexes :suggestable_title, type: 'string', analyzer: 'autocomplete'
+    end
+  end
+
+  def as_indexed_json(options={})
+    as_json.merge(suggestable_title: title)
+  end
+end
+
+Article.__elasticsearch__.client = Elasticsearch::Client.new log: true
+
+# Create index
+
+Article.__elasticsearch__.create_index! force: true
+
+# Store data
+
+Article.delete_all
+Article.create title: 'Foo'
+Article.create title: 'Bar'
+Article.create title: 'Foo Foo'
+Article.__elasticsearch__.refresh_index!
+
+# Search and suggest
+fulltext_search_response = Article.search(query: { match: { title: 'foo'} } )
+
+puts "", "Article search for 'foo':".ansi(:bold),
+     fulltext_search_response.to_a.map { |d| "Title: #{d.title}" }.inspect.ansi(:bold, :yellow),
+     ""
+
+fulltext_search_response_2 = Article.search(query: { match: { title: 'fo'} } )
+
+puts "", "Article search for 'fo':".ansi(:bold),
+     fulltext_search_response_2.to_a.map { |d| "Title: #{d.title}" }.inspect.ansi(:bold, :red),
+     ""
+
+autocomplete_search_response = Article.search(query: { match: { suggestable_title: { query: 'fo', analyzer: 'standard'} } } )
+
+puts "", "Article autocomplete for 'fo':".ansi(:bold),
+     autocomplete_search_response.to_a.map { |d| "Title: #{d.suggestable_title}" }.inspect.ansi(:bold, :green),
+     ""
+
+puts "", "Text 'Foo Bar' analyzed with the default analyzer:".ansi(:bold),
+     Article.__elasticsearch__.client.indices.analyze(
+      index: Article.__elasticsearch__.index_name,
+      field: 'title',
+      text: 'Foo Bar')['tokens'].map { |t| t['token'] }.inspect.ansi(:bold, :yellow),
+     ""
+
+puts "", "Text 'Foo Bar' analyzed with the autocomplete filter:".ansi(:bold),
+     Article.__elasticsearch__.client.indices.analyze(
+      index: Article.__elasticsearch__.index_name,
+      field: 'suggestable_title',
+      text: 'Foo Bar')['tokens'].map { |t| t['token'] }.inspect.ansi(:bold, :yellow),
+     ""
+
+require 'pry'; binding.pry;