Suggested addition: help-block support

[natelloyd]

http://getbootstrap.com/css/#forms-help-text

Hopefully I can look at this myself when I'm not under the gun of a deadline, but I'd love to be able to include 'help-text' in $options and prepend that before the closing div of the form-group.

Glancing through the code, it appears there may be some issues with errors being wrapped in a help-block span, so it may not be an easy change.