A preliminary pass at number_with_delimiter

This is more to start a conversation as well as provide a possible
single concrete implementation for other things that might go into
Lotus::Helpers:

- I18n
- HTML Safe concerns
I'd like to have one helper per group of responsibilities, not an helper with a single method. It makes application's code less noisy.

Example:
```ruby
module Formatters
  def format_number
  end

  # ...
end

module Converters
  def convert_temperature
  end

  def convert_currency
  end
end
```

Those above are just an example, we should be able to group methods properly.
I'm reporting this here as future reference.