[Feature] Add support for <output> in FormBuilder/FormHelper

The <output> element provides a semantic way to express that a particular part of the page is related to one or more <input>s. It can then be used as a target for various JavaScript connections between them.

Currently, as best I can tell, there is no built-in way of asking Rails to render one. Instead I need to manually add the element to my ERb files and associate the for= attribute by hand.

It would be nice if there was direct support. For example:

    <%= form.label :a_value %>
    <%= form.range_field :a_value, value: 5 %>
    <%= form.output :a_value %>

Would render as something like:

  <label for="formname_a_value">A value</label>
  <input type="range" name="formname[a_value]" id="formname_a_value" value="5">
  <output for="formname_a_value">5</output>