Fallback to `select` if array of `checkboxes` fail

* Allows fields that are arrays of strings to be passed to `<select multiple>`
inputs
* Update README.md
* Add coverage for Array of checkboxes
* Rescue `Capybara` errors and rethrow as Formulaic
* Check for presence of all options in ArrayInput
* Raise Formulaic::InputNotFound with useful message if not all options
  are present in either a select[multiple]'s options or as checkboxes.
* Ensure checkboxes exist with `:has` http://dev.w3.org/csswg/selectors4/#relational
* Extract `SelectInput` and `CheckboxInput`
 * Both are subclasses of `ArrayInput`. Now, `ArrayInput#fill` chains together
   successive calls to `SelectInput#fill` and `CheckboxInput#fill`, finally
   failing with an informative exception if neither inputs exist on the page.
* Enure that I18n's aren't humanized