Add tab-index support [work in progress]

This branch adds support for the `tabindex` attribute. In order to support this more gracefully, I've also broken Bootstrap and non-Bootstrap templates apart so that formFor now consists of 3 modules: 'formFor', 'formFor.bootstrapTemplates', and 'formFor.customTemplates'. This should hopefully make things easier on developers that want to use one or the other (or implement their own UI entirely).