title: mix phoenix.gen.html with map field type
body: <p>I am using Phoenix with Ecto 2.0. I was positively surprised that I can pass <code>my_field:map</code> to the generator and it correctly generates model with map.</p>
  
  <p>It also generated the view and the field is simply string input, which fails validation, because string is not a map.</p>
  
  <p>Is there a format for this string that will be converted to a map?</p>
  
  <p>I am aware that there is no standard way of editing maps with HTML. Just curious if it would be a good idea to parse the string automatically in generated code. This way generator would always produce working solution.</p>