Customizable FORMFIELD_OVERRIDES

Would it be useful addition to move default `FORMFIELD_OVERRIDES` and `formfield_callback` to floppyforms.**future**.defaults and allow providing alternate `formfield_callback` function with settings, ie: `FLOPPYFORMS_FORMFIELD_CALLBACK='myapp.formfield_callback'`?

Use case is when user want to customise globally floppyforms fields within modelforms (ie: not to use HTML5 inputs).

If this seems reasonable, I can work on it.

Start by locating FORMFIELD_OVERRIDES and formfield_callback in the repository, then trace how model forms obtain their defaults. Done means defaults are exposed through floppyforms.future.defaults and a setting can select an alternate callback such as myapp.formfield_callback.

Written by the indexing model from the issue text.