title: Tokenizing string for completion suggester
body: <p>want to build the auto complete functionality of an e-commerce website, using Completion Suggester.</p>
  
  <p>This is my Index:</p>
  
  <pre><code>PUT myIndex
  {
      "mappings": {
          "_doc" : {
              "properties" : {
                  "suggest" : {
                      "type" : "completion"
                  },
                  "title" : {
                      "type": "keyword"
                  }, 
                  "category" : { 
                      "type": "keyword"
                  },
                  "description" : { 
                      "type": "keyword"
                  }
              }
          }
      }
  }
  </code></pre>
  
  <p>Now, when uploading the advertisement I want the title field to be used for auto complete, so this is how I upload a document:</p>
  
  <pre><code>POST dummy/_doc
  {
    "title": "Blue asics running shoes",
    "category": "sports",
    "description": "Nice blue running shoes, size 44 eu",
    "suggest": {
      "input": "Blue Asics running shoes" // &lt;-- use title
    }
  }
  </code></pre>
  
  <p>Problem is, this way, elastic search only matches the string from beginning... i.e. "Blu" will find result but "Asic" or "Run" or "Sho" won't return anything...</p>
  
  <p>So what I need to do is to tokenize my input like this:</p>
  
  <pre><code>POST dummy/_doc
  {
    "title": "Blue asics running shoes",
    "category": "sports",
    "description": "Nice blue running shoes, size 44 eu",
    "suggest": {
      "input": ["Blue", "Asics", "running", "shoes"] // &lt;-- tokenized title
    }
  }
  </code></pre>
  
  <p>This would work fine... But how am I supposed to tokenize my field? I know I can split the string in c#, but is there anyway that I can do this in Elasticsearch/Nest?</p>