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
Idk if we should instantiate this builder here, because it's used only in two tests and this way it's been instantiated for all specs that use this setup. What do you think @flooberleitner ?