Update 'plaintext' field to pass W3C validation and replace <p> tag by <div> tag.

*  Remove the `for` attribute of the `<label>` tag when we use `plaintext` field with TwitterBootstrap3 framework to pass W3C validation
* Use `<div>` tag instead of `<p>` tag for `plaintext` field with TwitterBootstrap3 framework to get more flexibility.  E.G. You can now store `<ul>` tag inside your `plaintext` field.

See: https://github.com/Anahkiasen/former/issues/377#issuecomment-56032144 and https://github.com/Anahkiasen/former/issues/377#issuecomment-56032654