title: jQuery FormBuilder validate only required fields
body: <p>I am using the jQuery FormBuilder plugin. Link: <a href="http://formbuilder.readthedocs.io/en/latest/" rel="nofollow noreferrer">http://formbuilder.readthedocs.io/en/latest/</a>. Now, when a user submits the form, I want to make sure all the required fields (with an *) are filled out, else the form  won't submit (I will get an error message 'An invalid form control with name='xxx' is not focusable.'). There are many types of  input in the form, including text, textarea, dropdown select, radio and checkbox etc. How do I validate all of them?</p>
  
  <p>An example:</p>
  
  <pre><code>&lt;form-template&gt;&lt;fields&gt;&lt;field type="text" required="true" label="Text Field" class="form-control" name="text-1531271871930" subtype="text"&gt;&lt;/field&gt;&lt;field type="select" required="true" label="Select" class="form-control" name="select-1531271872850"&gt;&lt;option label="Option 1" value="option-1" selected="true"&gt;Option 1&lt;/option&gt;&lt;option label="Option 2" value="option-2"&gt;Option 2&lt;/option&gt;&lt;option label="Option 3" value="option-3"&gt;Option 3&lt;/option&gt;&lt;/field&gt;&lt;field type="radio-group" required="true" label="Radio Group" name="radio-group-1531271874156"&gt;&lt;option label="Option 1" value="option-1"&gt;Option 1&lt;/option&gt;&lt;option label="Option 2" value="option-2"&gt;Option 2&lt;/option&gt;&lt;option label="Option 3" value="option-3"&gt;Option 3&lt;/option&gt;&lt;/field&gt;&lt;/fields&gt;&lt;/form-template&gt;
  </code></pre>