Allow a project to mix syntaxes

[sunny]

The current solution relies on replacing the I18n.backend but does not allow a project to mix and match both syntaxes.

One solution would be to override load_file to switch syntaxes depending on the file name (.flat.yml, .locale.yml).

Another solution could be to to use different folders altogether so as not to collide with the simple loader:

I18n.backend = I18n::Backend::Chain.new(
  I18n::Backend::Simple.new, # Uses I18n.load_path
  I18n::Backend::Flat.new('config/translations/*.yml'),
)