Builder option 'force_help_block' to forcefully render help-block of inline-errors

Hi maintainers,

I did a small addition to your gem.
As I am using it to generate a remote form I needed the help-block spans be forcefully rendered (not just upon precence of help messages) so I could populate them upon AJAX feedback on the submit.
-> I added a builder option to take care of this.

I did not create an issue for this as I was just playing around a little at first (to see if I could manage that)

Let me know if this of interest for you or if you would require some additions/changes before you could accept the pull request.

Greetz,
Florian