Separate bootstrap (and upcoming zurb) integration setup

[carlosantoniodasilva]

We want to remove all if/else conditions in SimpleForm initializer template, mostly related to bootstrap and the upcoming zurb foundation integrations, so there are two possibilities:

• Move them to the bottom of SimpleForm initializer, as a separate configure block, to override the defaults.
• Move them to a separate file each, for instance simple_form_bootstrap.rb, to be copied to app initializers when running the install generator.

All this will allow us to keep better defaults in the initializer.

[nashby]

I think the second one is better and I can work on it. What do others think?

[rafaelfranca]

@nashby 👍

[josevalim]

A separate file seems ok.