Add Forme.default_add_blank_prompt for setting default prompt used for :add_blank option

This is the simpliest way to handle the default prompt value.  It
can be made more granular (e.g. per form or per configuration) later
if there is a need.
Support :autofocus option for setting autofocus attribute, similar to :required and :disabled
Support Form :values option, for automatically setting value attributes for inputs from submitted params