Add option to enforce translations

[Nerian]

Hi,

This PR adds an option to enforce translations from location files instead of falling back to humanizing attribute names.

[Nerian]

Also related to this is ruby-i18n/i18n#654.

The goal being to have this kind of errors:

I18n.t(
  :"role.name",
  scope: : "simple_form.placeholders",
  default: [:"defaults.new.name", :"defaults.name"], 
  raise: true)

Translation missing. Options considered were:
- en.simple_form.placeholders.role.new.name
- en.simple_form.placeholders.role.name
- en.simple_form.placeholders.defaults.new.name
- en.simple_form.placeholders.defaults.name