Use neutral defaults in generators

Simple form generators were adding error notification classes and
label classes configurations to simple_form's initializer which are
Bootstrap specific. To avoid this, this defaults were moved to
simple_form bootstrap initializer.

Fixes #861
Add wrapper mapping per form basis.

Fixes #931.
Merge pull request #932 from plataformatec/lc-neutral-defaults-from-base-generator

Use neutral defaults in generators
Changelog entry for wrapper mapping per form basis.
Merge pull request #934 from rcillo/add-form-wrapper

Add wrapper mapping per form basis.
Move changelog entry to the master section.

It was mistakenly added to the already released 3.0.0 section.