Arbitrary format options

Today I was struggling to find a way to pass arbitrary format options through the `humanized_money` helper. All I found in the code was a TODO that more options should be added soon. I think this is not the right way to do it since `money-rails` then must potentially mirror all future format options of the `money` gem.

My PR essentially lets you pass arbitrary format options, not just `:no_cents_if_whole`, `:symbol` and `:sign_before_symbol`. This will account for all options that may exist in the future. For example:

```ruby
config.default_format = {
  :no_cents_if_whole => true,
  :option_that_nobody_imagined_yet => :of_course
}
```

Cheers